Because Ricardo Montalban had appeared on the original series episode “Space Seed”, director Nicholas Meyer was not involved with casting him though he certainly had no complaints — “Khan is enough to tell you that this is a great actor,” he said. Most of Kirk’s crew were in place but Meyer was intimately involved with casting several new roles. He explained that what he was looking for was actors where he could see what the characters were feeling, even when they were not talking.
For Carol Marcus I wanted a woman who was beautiful and looked like she could think; a woman who was attractive enough that you could see why Kirk would fall for her, and at the same time somebody who could keep up with him. […] I loved Bibi Besch; I became very close with her, and I used her again in ‘The Day After’. She’s no longer alive and I bitterly regret it; she was a lovely human being, and a lovely actor.
Merritt Butrick is also tragically no longer alive. [As David Marcus] he not only had to be Kirk’s son, he had to be Carol’s son, so on a physical level I think what I liked was that his hair was the same color as hers but it was curly like Bill’s, so I thought, ‘Well, that’s plausible.’
Paul Winfield was an actor I had wanted to work with since I saw ‘Sounder’, and I thought, ‘Wow, what a lovely actor.’ There was no real reason for him to be the captain of the Reliant, other than my great desire to direct him in scenes! I knew he could do it, without any question.
The biggest casting coup was giving a young Kirstie Alley the role of Saavik. “She said as a child she wanted to be Spock and that she was so in love with the role that she wore her ears to sleep,” said Meyer.
She didn’t have to find the role; she didn’t have to work her way into it. She’d been living it somewhere in her head for years. There just wasn’t a contest. I don’t recall seeing another actor for that part who was as persuasive.
In addition to her instinctive understanding of the role, Alley brought another, slightly more definable quality to her role. “The thing about her is that she’s beautiful, but she also had a slightly otherwordly quality.”
She was also able to encompass that sort of flat unemotionality, but she’s basically a comedian. What I didn’t konw was that that flatness, like Leonard’s, frequently comes out of a kind of a deadpan. I realized that when I watched her doing it. Then, at the other end of it, there she was at Spock’s funeral, weeping. I remember somebody came running up to me and said, ‘Are you going to let her do that?’ And I said, ‘Yeah,’ and they said, ‘But Vulcans don’t cry,’ and I said, ‘Well, that’s what makes this such an interesting Vulcan.’
From Star Trek: The Magazine 3, 5 (September 2002)