Doctor Carol Marcus was a Starfleet science officer who served aboard the USS Enterprise from 2259. The daughter of Admiral Alexander Marcus and a mother with the maiden name Wallace, she had a doctorate in applied physics, specializing in advanced weaponry.
When Captain James T. Kirk set off for Qo'noS to find the traitor John Harrison, Marcus boarded his shuttle under the name Carol Wallace, using her mother's maiden name and claiming Admiral Marcus had assigned her. Spock deemed her credentials impressive but decided she was redundant, as he himself had been reinstated as first officer of the USS Enterprise. On the other hand, Kirk welcomed her aboard.
Spock confronted Carol about her name not being in the ship's records, so Carol apologized for the deception, explaining she had used her mother's maiden name to board the ship and examine the seventy-two experimental photon torpedoes her father had given Kirk to fire on Harrison. Spock informed Kirk about the deception, following Harrison's apprehension.
Marcus went with Leonard McCoy to examine a torpedo on a deserted planetoid. McCoy accidentally activated the torpedo and trapped his hand, but despite Jim's orders to beam her up, Carol deactivated the torpedo before the emergency beam-up became necessary. Opening up the dormant torpedo, Carol and McCoy discovered a cryogenically frozen man inside.
When questioned, Harrison revealed he was Khan Noonien Singh, revived to help prepare for war with the Klingon Empire, and that he had planned to smuggle his fellow Augments in the torpedoes before he was caught. Admiral Alexander Marcus appeared in the USS Vengeance, a warship designed by Khan, and began firing on the Enterprise. Carol Marcus revealed her presence to him in an effort to persuade her father to cease the attack, but Alexander simply beamed her over and carried on. When taken to the bridge, Carol smacked her father but he ignored her, claiming the cover-up was necessary.
Scotty deactivated the Vengeance's weaponry, giving Kirk and Khan time to fly to the ship and commandeer it. When they arrived on the bridge, Scotty shot to stun Khan while Kirk admonished the admiral for compromising Starfleet. However, one shot was ineffective to keep Khan down, and he promptly beat Scott and Kirk. He then turned to Admiral Marcus, who tried to flee into the turbolift: Carol tried to stop him, but he responded by breaking her leg. Then he grabbed her father and crushed his skull before her very eyes, and she responded by screaming in horror.
Khan beamed Carol Marcus, along with Kirk and Scott, into the Enterprise's brig. Kirk brought her to sickbay for McCoy to patch her up. Khan resumed bombarding the Enterprise, and the damage caused the ship to fall to Earth. Kirk went into the warp core and reactivated the engines, preventing the Enterprise from crashing, but at the cost of poisoning himself. Carol watched as Jim's unconscious body was brought to sickbay. However, McCoy realized Khan's blood could be used to heal him. He and Carol placed Kirk in one of the augments' cryotubes to put him in suspended animation. Spock and Uhura beamed down to apprehend Khan, after he crashed the Vengeance into San Francisco, and the blood transfusion proved successful.
Carol attended a memorial for those killed by Khan and her father, close to a year later. She continued serving on the Enterprise bridge as science officer when it embarked on the first five-year mission. (Star Trek Into Darkness)
- Smuggles herself onto the USS Enterprise via forged documents.
- Permanently assigned to the USS Enterprise for five-year mission.
"I'm Carol Marcus"
- - Carol Marcus formally introducing herself to James Kirk (Star Trek Into Darkness)
"I am ashamed to be your daughter."
- - Carol Marcus to Alexander Marcus on her father's actions (Star Trek Into Darkness)
- - Carol Marcus to Khan Noonien Singh before he breaks her leg (Star Trek Into Darkness)
"Dr. Marcus! Uh..I'm glad you can be part of the family."
"It's nice to have a family."
- - James Kirk and Carol Marcus on Carol joining the Enterprise crew. (Star Trek Into Darkness)
A young Carol Marcus was in an early script draft of the film Star Trek. She would have been represented as meeting Kirk during his adolescence and living near him.  Co-writer Roberto Orci explained, "We wanted to potentially lay down the foundation for [their] friendship [....] And then literally because of the nature of the introductory story and making sure that our core group of characters had the proper amount of story that they deserve, she went into the potential future draft." 
Introducing Carol Marcus in Star Trek Into Darkness brought some major implications to the movie, the prime-universe representation of the character having made a singular appearance – as the mother of Kirk's son, David Marcus, and a scientist involved in testing the Genesis Device – in Star Trek II: The Wrath of Khan. "What we didn't want to do was rush it in a way that felt too familiar and too predictable," commented Alex Kurtzman, regarding how he and the other screenwriters wrote the role into Star Trek Into Darkness. "The idea with Carol was introduce a character that implies a lot of things, but leave it open enough that anything could happen in the next movie." 
Showing Carol Marcus changing into her flightsuit, in Star Trek Into Darkness, was motivated by Director J.J. Abrams wanting to appeal to Star Trek's male fanbase, which is fairly large.  That the film would show Dr. Marcus in her underwear was heavily discussed during many story conversations between the screenwriters, as they contemplated how they were going to justify that. Orci said originally Kirk would have stumbled on Marcus changing as she prepared to open the torpedo in space, but budgetary limitations prevented them from setting the disarming scene in space: Orci acknowledged having her change into a flightsuit was not as necessary as showing her change into a spacesuit. 
The writers approached the subject from the point of view that, like in Star Trek II, Carol Marcus was again to be represented as bold in her interactions with Kirk, despite each of them being another form of their character than from The Wrath of Khan. "We figured, how do we harness the spirit of that in this scene," recalled Alex Kurtzman, "and that’s ultimately where we came to it from [....] It’s not something we went into blindly, and certainly we all sat in a room going, okay, we’re going to be criticized for this, but how do we justify this in a way that feels like it was thought about?" 
The alias Carol Marcus uses, Carol Wallace – besides referring to her mother's name – is likely a tribute to Janet Wallace.  That role was originally written as Kirk's love interest in early drafts of The Wrath of Khan and evolved into Carol Marcus in later drafts of the same film. (The Making of Star Trek II: The Wrath of Khan; Star Trek: The Magazine Volume 3, Issue 5) Both women were envisioned as strong-willed, intelligent scientists whom Kirk had previously been emotionally involved with.
Scotty actor Simon Pegg was thrilled to discover that Carol Marcus is involved in Star Trek Into Darkness. "Looking at it from the outside," Pegg recalled, "when I first saw her name in the script, it was exciting, 'cause Carol Marcus is a figure from Star Trek history."  Kirk actor Chris Pine once referred to the character, as written for Into Darkness, as "a really interesting part." (SciFiNow, issue 80, p. 23)
Casting the role
In Star Trek Into Darkness, Carol Marcus was played by Alice Eve. It was the people involved in the project that drew her to it, rather than any familiarity with Star Trek, which she didn't know much about before taking the role.  "I didn’t know who Carol Marcus was," she admitted. 
Alice Eve was likewise unaware, when she auditioned for the part, that it was the role of Carol Marcus she was trying out for. Although no full script was yet available, J.J. Abrams sent some scenes to her.  Before Eve received the scenes, Abrams asked that she prepare them. (Star Trek Magazine issue 172, p. 52) "I got a few pages of this character’s scenes and a scene that was kind of imagined, that wasn’t in the movie," remembered Eve. "It was just so well constructed and so well imagined and so truthful that I wanted to be part of this group of people that told stories in that way." The audition scene that is not in the movie was, in Eve's words, "an extended dialogue scene" between Carol Marcus and her father. The scene (whose plot points were included elsewhere in the film) was helpful to Eve in her understanding of Carol Marcus.  In fact, despite not knowing the name of the character she was auditioning to play, Eve got a distinct impression of the role from the collection of scenes, later stating, "I got a sense [that] [...] the journey of the character [J.J.] wanted was a girl who was struggling with her relationship with her father." Eve also consistently assumed that it was an already established character she would be playing, rather than a brand new one. 
Alice Eve proceeded to do as J.J. Abrams had requested by preparing the scenes. "I did that, and met with him, where we work-shopped the material," said Eve. This meeting was the first encounter between them, Eve never having auditioned for Abrams before nor even having crossed paths with him. "The process of working with him was incredibly rewarding for me, and enjoyable," Eve continued. "Then I spoke to him and his producing partner, Bryan Burk, afterwards." (Star Trek Magazine issue 172, pp. 52 & 54) Alex Kurtzman was another production staffer who witnessed the audition. He later recollected, "When we started auditioning Carol, in walked Alice, who was so wonderful and had this kind of lovely quality and brightness and intelligence. That inspired us to think about, 'OK, if we like Alice, then how are we going to craft the character around the actress that we like and make it familiar and not inconsistent with what you've seen before?"  Although it was by now obvious to Eve that the project she was auditioning for was Star Trek, Abrams didn't inform her that the character she was being considered for was Carol Marcus. "To be honest, that didn't mean anything to me anyway," Eve conceded, "because I wasn't overly familiar with the universe. He gave me a sense of who the woman was, what drove her, and what her motivations were. All of that gave me enough to create a sketch." (Star Trek Magazine issue 172, p. 54)
About ten days after Alice Eve's initial meeting with J.J. Abrams, he called her and asked her to do the forthcoming film. (Star Trek Magazine issue 172, p. 52) The news that Eve was cast in the part extremely excited Simon Pegg.  "I think I maybe got the job because I naturally speak very fast," reckoned Eve; she implied that the quickness of her speaking worked well in sync with Abrams' thinking, observing that he thinks "very fast." (Empire, Issue 287, p. 92)
Initial movie depiction
As Alice Eve naturally has an English accent (though can do a considerably convincing American one) whereas the original portrayal of Carol Marcus featured an American accent, there was "ample discussion," said Eve, as to which accent Marcus was to have in Star Trek Into Darkness. Ultimately, the Oxford-educated Eve was allowed to portray the character with her own accent, later quipping, "The association sometimes is that English people are more intelligent than Americans. Of course, it isn't true, but I think we decided to play into that stereotype... because she is such a super-brain." 
Once Alice Eve found out which character she had been selected to depict, Eve was happy to have been cast as Carol Marcus. This was because the vast differences between how she had been written in Star Trek Into Darkness and her previous presentment meant that Eve didn't have to copy that earlier portrayal.  Nevertheless preparing for the role by watching every episode of Star Trek: The Original Series' first season and Star Trek II: The Wrath of Khan, Eve took certain cues from the prime-universe version of Dr. Marcus, saying, "Oh, it’s information you have to carry, definitely. It doesn’t shape it, but it will maybe inform certain choices." Eve kept in mind that Marcus was originally depicted as being "cleverer than Kirk." The actress noted that she thus kept Marcus' "composure and her conviction and her thoughts, her belief in her intellect."
However, believing that the prime-universe Carol Marcus has "great composure" – brought about by age as well as by lost aspirations and relationships – Eve found that she had to temper this characteristic to play a younger, less experienced rendition of the character.  "This Carol Marcus that I was playing, I didn't feel she was tough," offered Eve. "I wanted to bring a vulnerability to her, because she was at the beginning of her journey as a woman [....] I wanted to bring that sense of hope and softness to her, along with the list of capabilities that are on the page. And she became mine at some point, I understood that there was a tradition, but I didn't hold too tightly to any of it because she was mine now." (Star Trek Magazine issue 172, p. 54)
Having spent much of her early twenties in further education, Alice Eve found she could relate to the highly educated Carol Marcus. "I was familiar with that kind of mind," Eve noted. "I think she has a specific motivation that was one that I understood and could sympathize with, so both resonated with me quite strongly." (Star Trek Magazine issue 172, p. 54) Eve wasn't entirely like the character, though. "I definitely knew that Carol was braver than I was when we started," Eve stated, "and more tenacious." (Star Trek Magazine issue 172, p. 55)
Hair Department Head Mary L. Mastro was conscious of styling Alice Eve's hair, for her role as Carol Marcus, in such a way as to be similar to that of the character's initial representation. "Alice doesn't look anything like the person who originally played that role," observed Mastro, "but we wanted to give her a sensibility that was similar, yet not have it be that period."  The hairstyle for the revised Carol Marcus was also influenced by a lengthy debate about the nature of hair in the future time period of Star Trek and the outcome of this discussion: the realization that J.J. Abrams usually tended toward stylistic minimalism in Star Trek. "So we used the famous Vidal Sassoon bob from the '60s to build a hairstyle," explained Alice Eve. "We think it showed a sense of youth, and was in keeping with the original imagery of the '60s show, but we modernized it and it added a clean efficiency to the line. It was that stuff we ended up falling back on, and J.J. was involved in all aspects. He was even involved with the color I chose to wear on my nails." (Star Trek Magazine issue 172, p. 55)
Alice Eve found that portraying the sexual chemistry between Dr. Marcus and Captain Kirk came naturally to herself and Chris Pine, mainly due to him having blue eyes that she found "really beautiful."  The actress also appreciated that, while Marcus took "great" support from her relationship with Kirk, their bond didn't define Marcus and that she "was a very determined woman in her own right, dealing with her own struggles." 
Performing the scene wherein Carol Marcus briefly appears in her underwear was fairly straightforward. When asked whether she found portraying the scene awkward, Alice Eve replied, "That's always an experience. I enjoyed working with everyone very much, I felt very comfortable in that environment and we'd all been training incredibly hard to, like, look like we were capable of living in space and saving the world. So, you know, I enjoyed the whole process of making that film, it was a good one."  The image of Marcus wearing only her underwear circulated around the Internet. There are mixed opinions about the scene, one criticism being that it undermined Marcus as a character.  Despite the brief shot turning out to be controversial and criticized by some as an exploitative and unnecessary sexualizing of an otherwise strong female character, Eve feels it was neither overly sexualizing her persona in the film nor inappropriate, given that Kirk is shown having sex with women and with his shirt off in both Star Trek and Star Trek Into Darkness. Eve has argued that not only was she fit when she filmed the scene but also that exhibiting the beauty of the female body in a strong character doesn't detract from such a character's strength or power. 
Carol Marcus is highlighted in a deleted scene from Star Trek Into Darkness, which explains the split universe's Carol was raised by her mother in England, thereby addressing why she has a different accent from her prime-universe counterpart.  The filmmakers specifically decided that, in the alternate reality, she was raised in London.  Editors Mary Jo Markey and Maryann Brandon also revealed Kirk's original first meeting with Carol, after Pike's death, was cut from the film and replaced during additional photography, which they felt added to Spock, and the audience's, initial suspiciousness about her. (Star Trek Into Darkness iTunes enhanced commentary)
Aftermath of first film appearance
Alice Eve has expressed happiness with her initial movie appearance as Carol Marcus, saying, "I really enjoyed the character I got to play."  Following the shooting of Star Trek Into Darkness, the actress clarified, "I know that I admire her tenacity, and her bravery. It's always great to play someone you can learn from [....] I hope I've learned from her." (Star Trek Magazine issue 172, p. 55) Eve has also expressed happiness that the alternate reality take on the character has been intriguing for online fans. "It's good to know that people are interested in Carol Marcus," she said. "I was really glad to be able to play her, with her intelligence and her quick wit with Kirk." (The A-Z of Star Trek, SFX Special Edition 61, p. 63) Eve also requested that there be an action figure showing herself as Dr. Marcus, remarking, "I would like one." 
Alice Eve has stated that, should she return as Carol Marcus in another film after Star Trek Into Darkness, she would "like to see Carol take the (captain’s) chair for at least an hour." The actress has been noncommittal about the story points relating to Marcus' relationship to Kirk, observing that the alternate reality leaves it open as to whether they eventually have a child, as in the prime universe. "I’d love any eventuality with Carol," Eve commented. "I love playing her. I love her strength and independence of mind. So I’d like to continue to see that." 
According to her dossier on the Star Trek Into Darkness App, Carol Marcus was born in 2233 in New York City. Her mother's name is given as June. She entered Starfleet Academy in 2254 and graduated in 2258.