NEW YORK • The Doctor, the shape-shifting, dimension-hopping hero of Doctor Who, may have travelled the cosmos for centuries, but for the earthbound humans who have portrayed him, the job has lately lasted about three years.

So it goes for Peter Capaldi, who came into the role on this long-running science-fiction series in 2014 and is now nearing the end of his journey. The Scottish actor said in January that the current season, which begins on Saturday on BBC America, will be his last.

Speaking by telephone from London, where he was taking a break from filming Doctor Who in Cardiff, Wales, he talked about his decision to leave the role and what he has learnt from his time on the series.

What is it like, in the midst of shooting a Doctor Who season, to get a few days of shore leave in the real world?

I'm sort of institutionalised when I'm filming. People lead me from my trailer to the studio. People ask me if I want cups of coffee. They give me a ride home, they pick me up in the morning. They tell me what I'm going to be doing the next day. After 10 months of that, you're like, "What crime did I ever commit to get into this really nice prison?"

The role of the Doctor seems to come with a built-in expiration date. Did you decide it was time to move on or did you just reach the end of your commitment?

Oh, no, they asked me to stay on. And I love this show. But I began to get worried about my capacity to deliver my best work. The schedule is intense and I began to wonder how many different ways I can say: "The time vortex is going to open up and destroy the entire universe as we know it, unless we blow up that model spaceship over there."

You have had nearly three seasons to play the character. What do you think distinguished your version of the Doctor?

First and foremost, he's not human. I think he struggles to create a version of himself that humans find easy to be around. He's always trying to save the universe, so if he upsets someone, that has to come with the territory. But then, later on, he probably gets a bit worried and wishes he could go back and say: "I'm sorry I upset you."

Was it your idea that he should also play the electric guitar?

We had just done my first season and I made a list of things that might be interesting to have a go at in the next season and one of them was guitar-playing. To my surprise, they went with it in quite a big way. I'd open a script and it would say, "The Doctor is playing Amazing Grace, Jimi Hendrix-style." I had a day where they said: "You've got to go buy the Doctor's guitar."

I thought he should have a Fender Stratocaster, but every time I tried one on, it looked like I was having a midlife crisis. Then we found this Yamaha guitar, which looks like someone had described a Fender Stratocaster to somebody else and made it without ever seeing a picture of it.

NYTIMES