She was aboard during the Enterprise's first fateful excursion beyond the galactic barrier, a journey that changed Gary Mitchell forever. It changed her, too, but this did not become apparent until much later, at Delta Vega. There, she began to develop psionic energy abilities like those Mitchell had displayed.
Dehner had been aware of her esper rating (089), aperception quotient (20/100), Duke-Heidelburg quotient (256), and general knowledge quotient (654895-109) being better than average in all categories. This was evidenced, since childhood, in superiority at guessing games, reading cards, and other activities. This was, in part, the basis for her vocational training as a psychiatrist. Participation in tests and studies of other related psionic-oriented beings were the subject of a thesis published by her in association with the College of Medical Sciences of the Tri-Planetary Academy and was, in fact, the reason for her posting to the Aldebaron colony.
When Mitchell fled captivity, he took Dehner with him into the hills of Delta Vega. Captain Kirk followed, determined to put an end to the threat posed by Gary Mitchell. Sensing Kirk's pursuit, Mitchell sent Dehner to intercept him; he hoped she would come to his opinion that mortals were highly insignificant. However, she retained enough of her Humanity to be appalled by Mitchell's actions and sacrificed her life to weaken Mitchell, an act that allowed Kirk to kill him.
Since NBC executives had found the depiction of Vina as an Orion slave girl in "The Cage" (Star Trek's first pilot episode) too raunchy, the role of Elizabeth Dehner was deliberately much less sexual for the following pilot episode, "Where No Man Has Gone Before". Herbert F. Solow observed, "Not even the most conservative of conservatives could characterize the female guest-star role [of Dehner] [...] as being too erotic." (Inside Star Trek: The Real Story, pp. 61 & 66)
In the script of "Where No Man Has Gone Before", Elizabeth Dehner is described as "past her mid-twenties, rather tall and slim, with a face that could be beautiful if she cared to make it so. She doesn't." The character was also scripted as having the middle initial "M," though this is not established in the episode's final version. Another facet that was scripted but not evident in the installment's final edit was that she was to initially appear horrified by Gary Mitchell showing her that, like him, she too had been transformed (with her eyes having turned metallic).
Dehner was played by actress Sally Kellerman. Director James Goldstone remembered, "We read a number of other actresses for the role played by Sally Kellerman. I, along with Gene [Roddenberry] and whoever else, did cast Sally, but again, Sally went with [Mitchell actor] Gary [Lockwood] beautifully. There was a marvelous kind of physicality to her." (The Star Trek Interview Book, p. 106)
Unlike Gary Lockwood, Sally Kellerman had no problems with the silver contact lenses required for the role. (Inside Star Trek: The Real Story, p. 80) She did, however, have a problem with Dehner's uniform. Due to embarrassment over the fact that the tight costume was particularly close-fitting around her crotch area, Kellerman was given a "space clipboard" prop that she held close to her, covering up the problem, and was shot from the waist up, whenever possible. Also, because she at first speculated that women of the future would not wear brassieres, she initially suggested that Dehner not wear one, but James Goldstone persuaded the actress to change her mind. (Inside Star Trek: The Real Story, pp. 83-84) Kellerman ultimately thought she looked somewhat "pudgy" in the costume. She stated, "You know, I never had to wear anything like this before." (Starfleet Access for "Where No Man Has Gone Before", TOS Season 1 Blu-ray special features)
Denise Okuda cited Elizabeth Dehner as one of many "strong" and "professional" women characters that "stood their ground" and were often conceived by Gene Roddenberry. (Starfleet Access for "Where No Man Has Gone Before", TOS Season 1 Blu-ray special features)
Dehner's biographical readout lists her age as 21, as of 2265 (stardate 1089.5), suggesting a birth year of 2244. It also lists her height as 5'2" and weight as 116. However, it is possible that the readout was a medical profile taken during her last examination, when she was 21, so she could very well be older during the events of "Where No Man Has Gone Before", as suggested by the script.
While not typical, many scientists and clinicians hold doctorates in both medicine and a related field, such as biomedical engineering – which makes Dehner's file, listing she had a PhD, neither inconsistent with real-world conditions nor necessarily a production gaffe. It is unknown in what field Dr. Dehner earned a PhD.