In the new film “Inside,” Willem Dafoe goes it alone as an art thief who breaks into a billionaire’s luxury penthouse with only seven minutes to get in, snatch a few multimillion-dollar paintings, and get out.

While director Vasilis Katsoupis’ “Inside” is a heist flick, albeit a classy art- and architecture-themed one, things very quickly go from bad to worse.

The hacked alarm code doesn’t work, and failsafe measures lock down the apartment tighter than a bank vault. Dafoe’s character Nemo is trapped with no way out. The penthouse is his prison.

“I saw it as an opportunity to have certain performance and storytelling and filmmaking challenges that were very specific,” Dafoe says of his attraction to the screenplay during a recent video call. “The fact that there were no conventional scenes. The fact that I knew it had to be shot in chronological order was interesting. The fact that it’s basically one actor.

“All those things were interesting to me,” he says.

About that “one actor” comment – remarkably, it’s true. “Inside,” but for two short dream sequences, is entirely a one-man show as Dafoe goes through Kübler-Ross-like stages of being trapped inside the luxury penthouse: Denial, anger, discovering the water in the fish tank is not good to drink, depression after eating the brightly hued resident of the aquarium, and accepting you might die like a modern-day pharaoh, surrounded by fabulous art inside this tomb with a view.

How long is Nemo trapped? It’s hard to say exactly, but judging by beard growth and general dishevelment, it’s at least several months.

Dafoe says he doesn’t know either but then corrects himself.

“I’ll tell you how long I was there,” he says, laughing. “Thirty-two shooting days.”

In an interview, edited for clarity and length, Dafoe talked about what it was like.

Q: That challenge, of an almost solo performance – what was it like to work that way?

A: You know, the truth is I don’t remember. In the respect that like the inanimate objects and the art and the space – and working quite closely with the crew and all the departments – I am like an object in the space. I never felt that alone.

And also because it’s not a conventional performance I’m not thinking about performance. I’m thinking about accomplishing things. I’m thinking about building a tower. I’m thinking about opening the can in a way that makes sense without a can opener. I’m thinking of finding what’s beautiful in the surveillance footage.

These are things I’m playing with. I’m not seeing acting. I’m not thinking about that. I’m doing things.

Q: Is there a freedom in that kind of work? Experimentation in trying different ways of moving through this space?

A: Yes. Yes, yes, yes. I’d say the majority of the film was sort of invented in the place when we were shooting, which was an interesting way to work. And sort of open-ended, where our actions are taking us is where they’re taking us. There’s no going back. No anticipating what happens at the end.

“We’re really kind of feeling our way along, and we’re afforded that luxury because we’re shooting in chronological order. Because we’re going to trash the house and there’s no going back.

Q: As time trapped inside passes, the art seems to matter less to Nemo. How did you view his psychological journey vis-à-vis his appreciation of the art?

A: When he gets stuck, his first preoccupation is how to survive. And then with time, once he solves, roughly speaking, those problems, he starts to deal with his surroundings. He starts to interact with the art. He starts to think about it. He starts to even work with it. And that process is interesting.

The fact that the same pieces of art at the beginning mean something totally different than they do at the end I like very much. Because that explores the idea that the inherent value of something depends on its context. It’s a simple concept, but we always assume that we give value judgment to everything, and it sticks.

And we forget that in another context it can have a completely opposite value. Something that is very desirable in one situation becomes a horror in another. And this luxurious place that many people would like to have for a home becomes this horrible, cold prison. How’s that happen? That’s interesting to me.