SPOILER ALERT: Do not read if you have not watched “Star Trek: Discovery” episode 6, “Lethe.”

As Ambassador Sarek, James Frain plays one of only two characters on “Star Trek: Discovery” to have appeared in a previous version of “Trek.” (The other is Rainn Wilson’s Harry Mudd.) Although Sarek showed up in the first two episodes of the new series, he did so mostly just to service backstory and help his foster daughter Michael Burnham out of a scrape. But Sunday night’s episode “Lethe” dove deep into the psychology of a fan-favorite character, showing him at a pivotal moment in his personal history as he was forced to choose whether to allow Burnham or his biological son Spock to suffer a crushing disappointment.

Frain spoke with Variety about Sarek, Burnham, and playing a fan-favorite character who keeps his emotions under wraps — most of the time.

Is playing a Vulcan different from playing a character whose emotions are more visible?
Well, that’s the challenge. When I dug around in the canon and the backstory of the character, the idea of Vulcans — there’s a sort of origin story. Apparently they were a very passionate and expressive species, and they had a catastrophic nuclear war on their planet and they came to the conclusion that that war occurred because of these out-of-control feelings that need to be repressed and managed. They built a culture based on logic so that nothing like that could ever happen again. So in their prehistory, there’s this massive repression. I thought that was really interesting, how that can be handed down from generation to generation. Yet there is for all of the Vulcans a conflict between something inner that has to be denied and a code of conduct that is absolutely rigorous. But there are opportunities for exploring something else. Sarek married a human. So something’s going on there that has never been really dug into.

Why is he attracted to humans in a way that other Vulcans seem very much not to be?
I think that’s something that I’m still exploring. It’s a very rich theme of the character — the tension that he has within himself between the person that he’s supposed to be and his beliefs, which are quite radical compared to any other Vulcan who we meet. That’s sort of an unanswered question right now. It’s something that we’re exploring this season and will continue exploring in seasons to come. And it does give me a lot of playing range between these opposite poles.

Sarek has appeared in multiple iterations of “Star Trek” over several decades, played by multiple actors. Was that a factor when you were figuring out your approach to him?
Mark Lenard performed most of who Sarek is. He’s a beloved character in that incarnation, and I felt like I had to honor that. I felt like I had to provide something that could conceivably lead to where he ends up in the [original] series. I didn’t want to get locked up in imitation. I don’t think that’s helpful. It was more like, “This is where he’s going. How did he get there and where did he come from?” But I also thought it was okay to explore other avenues, because sometimes people change in their lives, and sometimes they don’t.

How do you keep from making him too robotic or letting too much of his emotion surface?
It’s tricky. Too much of one is boring. Too much of the other is out of character. It’s interesting to play. It’s very challenging. I never know really if I’ve got it right. But that’s always the question in every scene, every beat — am I calibrating this correctly?

How did you prepare for the scene where Sarek speaks to the leader of the Vulcan Expedition and is forced to choose between Spock and Burnham?
The scene is a kind of “Sophie’s Choice.” He knows immediately that someone is going to get hurt, that someone is going to have to be sacrificed, and that this is an intolerable position that they’ve put him in. He’s at a loss for what the right thing to do is. It’s a really interesting beat for that character. We’ve always seen the Vulcans as people who have answers. And here’s a situation where he doesn’t have an answer. He’s talking to a kind of father figure, and the father figure is being patriarchal, and [Sarek] is being put in his place. He’s being punished, and he reacts in a way where they catch him being emotional, and he has to rein it in. But for a beat we see him totally out of depth. It’s a cruel thing that they do to him. I felt for him a great deal.

What’s the state of his relationship with Burnham now?
After it ends, she comes to him in his hospital bed and tells him, “This happened. Do you remember?” And he’s like, “Nope!” She’s beyond exasperated and angry at him. She wants this connection, and he just doesn’t know how to do that. He’s just not at the point where he’s able to handle it.