(written from a Production point of view)
Stewart is the only actor who appeared in the classic 1953 science fiction film War of the Worlds to have a credited speaking role in a Star Trek production. Stewart played a USMC captain in that film adaptation of H.G. Wells's story. One other Trek performer, David Sharpe, played a looter in that film, and was an extra (playing a security crewman) in a TOS episode.
Several months prior to the airing of "Charlie X", Stewart could be seen with his co-star from that episode, Robert Walker, Jr., in an episode of The F.B.I., a series starring Stephen Brooks. He went on to appear in two more episodes of The F.B.I., working with the likes of Ken Lynch, Joanne Linville, Fritz Weaver, and Jason Wingreen). Stewart's other acting credits include guest appearances on My Favorite Martian (starring Ray Walston), Batman, Bewitched, The Invaders (with Ken Lynch), The Brady Bunch, The Invisible Man (with Nehemiah Persoff), and Knots Landing (with Allan Miller).
In addition to episodic television, Stewart had small roles in a few made-for-TV movies during the 1980s: The Miracle of Kathy Miller (1981, with Brad Blaisdell and John de Lancie), Grace Kelly (1983, with Marta Dubois, Walker Edmiston, Salome Jens, Paul Lambert and William Schallert). Stewart can also be seen in such feature films as War of the Colossal Beast (1958), and The Hollywood Knights (1980, featuring Gary Graham).