It is strange to sit down with Fionn Whitehead only days after watching him in Bandersnatch, the choose-your-own-adventure Black Mirror episode that appeared on Netflix just after Christmas. As a viewer, you’re given the chance to make his character, Stefan Butler – a video game developer in the early 1980s whose project goes very wrong indeed – do all sorts of horrible and distressing things, directing the story, and his fate, with a series of sadistic clicks. If, of course, that’s the direction you decide upon. Perhaps you were kinder than me, and let it play out to only moderate doom. Or perhaps not. At one point, when a particularly gory option presents itself, Stefan looks into the camera and sighs, “Oh God, really?”, before he goes about the bloody task.

Whitehead is 21, and Stefan is already his second big talking-point role, after he played British soldier Tommy in Christopher Nolan’s harrowing 2017 epic Dunkirk. Whitehead is aware of the hype around Bandersnatch – there are countless forums online attempting to map out the many courses the story could take, using a series of increasingly dense flowcharts – but the brunt of the fuss has passed him by. “I’ve noticed through my friends and family,” he says. “I got a text from a friend saying, ‘Mate, you’re trending on Twitter.’ I didn’t really know what it meant. He adopts a withering tone. ‘It means apparently the TV show is good, dad.’ I was like, ‘Oh, cool.’”

Click bait … viewers of Black Mirror: Bandersnatch choose the fate of Fionn Whitehead’s character, Stefan. Photograph: Netflix

He is not on social media; his official Instagram and Twitter accounts are inactive and bare, and exist only to stop someone impersonating him. “It’s not my bag,” he shrugs. “I find it too easy to fall into the trap of relying on other people’s opinions to feel gratified. So anything I can do to reduce that risk is probably a good idea.”

He may be somewhat digitally averse, but he was a big Black Mirror fan before he got the audition. “My agent knew I would jump at the chance to be involved,” he says. He knew nothing about the episode when he went up for it, and was given only a single scene to prepare. “One that isn’t even really in it any more. It was sort of the ashtray scene, but a lot of things have changed in the script since I auditioned.” He got the job, not knowing that it was a job with a twist. “Then I went to meet Charlie [Brooker] and Annabel [Jones], and just before I met them, they sent an email along the lines of, ‘It’s a ‘choose-your-own-adventure’ thing.’ I was like, what? That doesn’t really make any sense.”

Brooker has said that Bandersnatch, with its many paths and endings, took roughly the same amount of work as four regular episodes of Black Mirror. Whitehead says that even the script was difficult to read; it was sent to him in Twine, software more typically used for writing games. “You spend a lot of time reading the script, thinking about the different choice paths, which added another level of complexity to the character arc.” He reckons the average feature film takes two or three months to shoot, without alternative storylines being made on top of each other. “But we shot the whole thing in six weeks, and it was four and a half hours’ worth of content.”

Out of the box … Fionn Whitehead. Photograph: David Levene/The Guardian

He found it helped not to overthink it. “Stefan is totally unaware that someone else is controlling him until very close to the end of the film,” he explains. “Depending on what path you take, and all the rest of it. During the shoot, I tried to put that bit to the back of my mind, which is a hard thing to do when you’re shooting a scene that is a variation of a scene, almost the same, but very slightly different. A lot of reshooting a scene where the words would be slightly different, and shorter maybe. But you’d have to have it in the back of your mind that it’s like a deja vu thing, of having done this action before.” Miraculously, it all comes together, and critics have responded well to its innovations, no matter how unsettling it seems for the viewer to be handed responsibility for a character’s most terrible life choices. “It was definitely an intense experience, but I loved it,” he says.

Whitehead is as closed about his personal life as you’d expect from someone who had already sworn off social media as a teenager: “I just enjoy my privacy. I think it’s important to have a life of your own, outside of work.” (His Bandersnatch co-star Will Poulter announced he was stepping back from social media in the wake of comments about the episode.) He is clearly less comfortable talking about himself than he is about what he’s been working on. He grew up near Richmond, and picked up a taste for acting at primary school. “I just remember making my friends laugh and being like, that’s great fun,” he says.

I find it too easy to fall into the trap of relying on other people’s opinions to feel gratified

It’s ironic that he started out wanting to amuse people when his roles so far have all been … “Incredibly dark? Mmm, I know,” he smiles. “That came later.” He thought about playing rugby professionally for a while, or being a musician. He comes from a family of musicians and can play guitar and drums, “but nothing amazingly. By the time I was 14 or 15, I think I was pretty set on acting.” After sixth form, he auditioned for a number of drama schools, and was rejected by every one. It’s a funny state of affairs now, considering how the last couple of years have gone, but it must have knocked his confidence at the time. “Of course, but you just have to learn to deal with it, I suppose. It’s part of being an actor, getting rejections. I think that the quicker you learn how to deal with them, the more able you’ll be to take them next time they come.”

He got a job in a coffee shop and went to open casting calls “with my terrible homemade CV”, preparing to reapply for drama school the following year. But just as he was about to start auditioning again, he was cast in ITV’s supernatural drama Him. “I never did my Ucas again.” Him was followed by Dunkirk, a blockbuster that was hugely successful and critically acclaimed. It all sounds like a bit of a fairytale. “It was very quick, yeah,” he says. “Very quick. I landed on my feet, I suppose.”

Acclaim … Fionn Whitehead on the beach in Dunkirk. Photograph: Allstar/Warner Bros

Whitehead is self-aware and polite enough to fall back on modest talk of “lucky” auditions and “lucky breaks”, but all of his roles share a certain impressive haunted intensity, and he is very good at doing distraught and distressed. After re-creating the horrors of Dunkirk, he starred alongside Emma Thompson as a dying teenage boy in The Children Act, an adaptation of the Ian McEwan novel. In Bandersnatch, Stefan is driven to breaking point by his endeavours, and the results aren’t pretty.

Despite the successes of the last 18 months or so, Whitehead says he doesn’t get recognised much. Perhaps because he’s much brighter than the damaged, shellshocked characters he usually plays. “Probably, yeah,” he says with a smile, and counts off his repertoire on his fingers: “I’m not looking sick, sad or angry.”

With its sneaky release, Bandersnatch ended up being one of the highlights of the festive TV period. Does he think there’s a future for interactive storytelling? He offers a cautious response: “There definitely could be. But it’s a very complex thing to do. So who knows? I’m sure people will try.” Whitehead has only watched the episode once, with his flatmate. They played it through a number of times, “but I didn’t do all of the choices”.

Surely he knew which paths would lead to where? “I had a vague idea, but I didn’t know,” he says. “We got almost every ending before we stopped, because it was getting quite late.” Of all the endings, though, they missed out on the one he liked best, the one that offered a little light in the dark. “The TV set ending, when you go to jump out of the window and an AD runs up, and says, you’re not supposed to be doing that. I found that really fun to shoot, that bit. I kept wanting to crack up.”