Most days, in between his hours spent in make-up and his first scene on the set of A Different Man, Sebastian Stan would walk the streets of New York in the prosthetics that made him look like his co-star, Adam Pearson. Pearson has neurofibromatosis, a genetic mutation that produces benign tumours that distort his facial features, surrounding them with lumps of skin. Stan, whose matinee idol looks gave him an entrée to the gilded cage of Gossip Girl and later, the Marvel universe, found people were self-consciously looking straight through him. Either that, or nudging each other to take a picture.

“Obviously, when you’re out in the real world, people are not as good liars as they are in front of a camera and you get a real reaction,” says Stan. “It was really important to me in understanding, or trying to understand as best I can, what it might be like to walk in the shoes of someone with neurofibromatosis, but also just being sidelined and alienated as someone who looks different. Unfortunately, it was very lonely out there.”

With his disfigured face, he found he stood differently, walked differently, and communicated differently. It is a world away from his other current role as the young Donald Trump in The Apprentice which, despite being the talk of the Cannes Film Festival, remained without a US distributor for months after Trump’s people sent out a round of letters threatening legal action against anyone who touched it. That could not change the fact that this is very much Sebastian Stan’s year. When The Apprentice was unveiled, his performance in A Different Man had already won him the best actor award at the Berlin Film Festival; it is also an outsider Oscar tip.

A Different Man, however, isn’t the kind of film usually embraced by the Oscars. It is a sharp, funny but ultimately tragic social satire about appearances, identity and personal integrity, shot over just four weeks, which Stan describes as “more like a European film: elevated and intelligent”.

He plays Edward, a severely disfigured, cripplingly shy aspiring actor. When Edward is offered surgery that will transform him into something else – namely, into Sebastian Stan as we know him – he jumps at the chance to remake himself. With the confidence of a new face, he might even manage to win the heart of his beautiful neighbour Ingrid (Renate Reinsve), an aspiring playwright.

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The universe betrays him, however. When he returns from his surgical transformation as a handsome fellow called Guy, he realises that he is still as awkward, spiky and unlikable as ever. A brief encounter with Ingrid is a hollow disappointment. Later, still trying in vain to become an actor, he discovers that Ingrid has written a play about her former friend Edward. He fails to nail the role, however, because he doesn’t look the part. Instead, the director casts a man called Oswald, played by Pearson.

Oswald looks exactly like his old self, but is ebullient, personable, charismatic and confident. He is, in short, everything Edward wanted to be.

Writer-director Aaron Schimberg’s earlier feature, Chained for Life, also starred Pearson as a much more cowed, retiring character. Working together, he says, inspired him to write a character with the same features but a personality closer to Pearson’s own. Schimberg himself has a cleft palate.

“So I have my own experience of being stared at, mocked, laughed at, growing up different. That was what I was drawing on, writing the script. And then who I’m going to cast in that role or these roles is another question I’m exploring.”

Historically, he points out, people living with disability have been played by conventional actors in masks. “Especially in stories that are sympathetic to disabled people, like The Elephant Man, trying to muster up some empathy. The movies that cast actual people with disabilities tend to be horror movies or more exploitational. I wanted to play with these ideas. Adam is not the sad person we know from cinema and I wanted to honour that.”

Pearson fell into acting when director Jonathan Glazer was casting a character with facial disfigurement for Under the Skin, a disturbing 2013 sci-fi starring Scarlett Johansson as an alien visitor to Earth. Pearson has a degree in business studies; he knew Glazer’s name as the doyen of British advertising.

“I’d written papers on the guy’s work in advertising and marketing, because he is a genius,” Pearson says. “So I was like OK, let’s throw your hat in the ring! I don’t know, it may be a bit of a laugh. And then it all got really serious really quickly.”

I think there is something very relatable to the film, because there are elements about identity and self-acceptance that we’re all facing.

Sebastian Stan

He and Stan were not on set together when Stan was made up – “which I think is a missed opportunity for an amazing press photo,” says Pearson - but they talked long and deeply about living with his condition before the shoot began.

“We were talking about how he could inhabit the character of Edward. And what I said to him, to give him a little something to hook on to, was that the two easiest ways to lose your anonymity in society is to have a disfigurement of some kind, or to be a celebrity.

“Because, either way, you become public property. The whole world feels entitled to know what is going on, where are you going, what are you doing. Tell me about you!” People in pubs, fortified by a bit of Dutch courage, will ask him to give an account of his life. As a hot celebrity, that was something Stan recognised.

“Invasion is something I’ve grown up being quite accustomed to,” Pearson continues. “And then I suppose it becomes a whole lesson in discernment. Who do you want to tell your story? Is this a curiosity I can answer, or is it dipping into disrespectful vaudevillian nonsense?”

Stan says he avoids some of the barrage of attention that comes with fame by staying off social media. “I’ve chosen to live in the real world and you know, in the real world people are very interesting with how they talk to you. If anyone’s going to come and say something to you, they’re going to have to say it to your face.”

As he discovered walking up Broadway in his mask, that doesn’t deter everyone. On the other hand, everyone knows how it feels to be judged. “I think there is something very relatable to the film that everyone can draw from because there are elements about identity and self-acceptance that we’re all facing,” he says.

We all want to be accepted as our authentic selves. But what is that self? Is it fixed?

“This is an exploration of identity, I guess,” says Schimberg. “Specifically, the question of how much of our identity is based on the perceptions of others.” He is also asking how malleable that identity can be. Edward is stuck with himself. “But I know people who have changed their personalities drastically, who have made it their mission to become a different kind of person,” he says. “That person who becomes someone else: are they playing a part? Or have they changed something deep inside?”

It would take more than just one movie to unravel that one.

A Different Man opens in cinemas on October 24.