Interviewing actors usually involves asking them to remember things: lines spoken, expressions pulled, performances given weeks, months or even years ago that are only now seeing the light of day. But instead of fondly reminiscing about his latest project, Asa Butterfield is desperately trying to envisage it. The Sex Education star is about to appear in Second Best, a play about the boy who came agonisingly close to being cast as Harry Potter in the film franchise. It’s his first week of rehearsals, and Butterfield isn’t merely figuring out how to play the part, he’s also trying to predict how scared he’ll be while doing it: in an extremely bold move, this 90-minute one-hander will be the actor’s theatrical debut.

It is an especially ballsy choice when you consider that theatre “has always terrified” Butterfield. “Standing on stage in front of hundreds of people without being able to say, ‘Cut! Can we try again?’ is sort of ‘eurgh!’” he says, sitting in the middle of a spartan rehearsal space in north London. He tries to visualise himself in the wings before the first performance. “I’m going to have stomach-churning anxiety, undoubtedly.” Is he someone who tends to ruminate on that sort of thing? “Yes,” he sighs instantly, with the knowing weariness of a chronic overthinker.

Butterfield may have zero theatrical experience, but when it comes to screen acting he’s probably the most seasoned 27-year-old around. Having starred in his first film when he was 10, by his teens he was leading Martin Scorsese movies (Hugo) and acting opposite Harrison Ford (in the sci-fi Ender’s Game). But it was as gawky, gaffe-prone schoolboy Otis Milburn – who finds himself solving his classmates’ intimate issues using wisdom gleaned from his sex-therapist mother – in Netflix’s smash-hit series Sex Education that he became properly renowned and beloved.

The show, which concluded in 2023 after four seasons, was known for its hyperreal transatlantic style, fusing bone-dry British humour with a classic American high school movie aesthetic (it also featured a bracingly progressive and rather far-fetched candour about sex that is typical of neither nation). As the resident virgin nerd, Otis’s look was largely brown and eye-wateringly 1970s: stripy polo shirts, chestnut cords. So it is slightly disconcerting to encounter him in a similar get-up today: brown trousers, a brown and beige jumper. Did he channel himself into the role? “Oh 100%. I really like playing awkward characters. I find awkward humour very funny, very endearing. And so I just cranked that up for Otis.”

On the one hand, he did start the interview by knocking over a cup of coffee (he absolutely did mop it up himself, which neatly doubled as an illustration of the ego-lite, luxury-free reality of putting on a London theatre production). On the other, the actor is clearly miles more comfortable in his own skin than the character he is now synonymous with. It makes sense, therefore, to hear that Otis was part-Asa, part-Mr Bean, with Butterfield drawing on the patron saint of ungainly goonery’s “bumbling, stumbling physicality” for the role.

“I was 25 and done with playing a teenager. We were all very ready to move on,” he says about the show’s conclusion. That said, he didn’t necessarily foresee a solo theatrical run in his immediate future, and was initially doubtful when he was approached for Second Best. But the fear factor was part of the appeal – here was an opportunity to really stretch himself professionally. TV favours acting that is “small and naturalistic”, a mode that Butterfield quickly discovered doesn’t really fly on stage. (He’s also learning how to bounce off the audience; he and Michael Longhurst, the director, both liken the monologue-heavy show to standup.) The other reason he felt compelled to take the job was the “beautifully written” script. “I loved it. And I didn’t want anyone else to do it. I really connected with it on multiple levels.”

In fact, the overlaps between the play and Butterfield’s real life are quite spooky. Second Best is adapted from award-winning French writer David Foenkinos’s tragicomic novel of the same name, which follows a (fictional) 10-year-old Londoner called Martin Hill, who is scouted to audition for the part of Potter at the turn of the millennium. He’s the favourite, until Daniel Radcliffe pips him to the post at the last minute. The trauma of this disappointment taints the rest of his life, as the global ubiquity of the film franchise renders the memories of his rejection inescapable; Radcliffe’s glorious, breathlessly chronicled career feels like a relentless slap in the face.

The novel weaves together fact and fiction, but the producer who discovers Martin is real: David Heyman was an early champion of JK Rowling who was instrumental in finding the right boy to play Potter. Less than a decade later – in “an eerily similar series of events” – Heyman would cast another child in a leading role in a major motion picture: Butterfield. He played the son of an SS officer who befriends a Jewish boy in a concentration camp in The Boy in the Striped Pyjamas. Like Martin – who in the book is spotted while accompanying his prop master dad to work on the set of Notting Hill – Butterfield was also plucked from obscurity. The Londoner was eight years old and attending an after-school drama club with his brother – “My mum thought it would be good for me to come out of my shell and meet new people” – when he caught the eye of a visiting casting director who “wanted me to audition for things, and I did”.

Unlike Martin, Butterfield got the part. And then another, and then another. Looking back, however, he doesn’t view his career as a child actor as much of a triumph. “When you’re a kid, the craft of acting is a bit lost on you,” he says. He thinks children perform with an “intuitiveness”, something that means a proper grasp of the job can be actively detrimental to the performance. “You don’t want to think [as a child actor] because when kids are thinking about things, you see it on their face.”

During the filming of The Boy in the Striped Pyjamas, the plot and many historical details of the Holocaust were kept from Butterfield, partly to avoid disrupting the innocence of his performance, but also because the subject matter was so deeply horrifying. “You don’t need to know all those details as a kid. It was at times traumatic enough making that movie. The production team protected me in all the ways they could, but there are scenes in that film that I remember coming out of and being like: I don’t want to do this any more. Thinking back on it, I’ve got little sisters who are 12 and 15, and I wouldn’t want to put them through that.”

Understandably, The Boy in the Striped Pyjamas didn’t convince Butterfield to pursue acting as an adult. In fact, it wasn’t until he was cast in Martin Scorsese’s Hugo – a fantasy about a boy who lives alone in a Paris railway station – that he “fell in love with acting – but also film-making”. Although that experience wasn’t entirely devoid of unsettling moments. Every weekend, Scorsese would give Butterfield a movie to watch, “usually something quite old. At one point he gave me David Cronenberg’s The Fly. I was 13 and was like: ‘Why did you make me watch that?!’”

Despite the impressive CV he was building, Butterfield’s decision to continue acting was accompanied by a strong suspicion that he didn’t know how. “Between the ages of 14 to 19 I suddenly had to relearn how to act, because I could no longer rely on being in the moment. You end up overthinking what people expect from you, or how to analyse a character or break down a scene.” It was around this time that Butterfield, like Martin, did narrowly miss out on a massive role: in 2015 it was widely reported that he was in talks to replace Andrew Garfield as Spider-Man; the part ultimately went to Tom Holland who has gone on to become one of the defining stars of Hollywood today, not to mention engaged to his Spider-Man co-star and fellow A-lister Zendaya.

Speaking today, he isn’t just philosophical about the opportunities that didn’t go his way – “You have to learn that it’s rarely in your hands” – but refuses to view any element of his career through the prism of failure. “At this stage I don’t think of it like that because I’ve had some really great successes.” Still, rejection is rejection, and from the outside an actor’s life can seem brutal. Is he really that sanguine? “I’m not saying it’s easy. I still have roles where I go: ‘I really wanted that!’ And having that acceptance is a lot easier said than done. But it’s something you constantly get better at.”

Perhaps Butterfield wouldn’t be quite so cheery if he hadn’t signed up for Sex Education – which he very nearly didn’t. There was no need for him to audition for the show: in the meetings leading up to casting, Butterfield says the producer and director kept telling each other they needed “to find an actor like Asa Butterfield. And then they said: ‘Well, why don’t we go to Asa Butterfield?’” But when he read the script, he “didn’t see it. I thought, I don’t know how they can turn this hook into a multi-series show,” he says, referring to Otis’s sex-advice sideline. Such was his hesitation in accepting that the rest of the cast – which included the then-unknown young actors Emma Mackey, Aimee Lou Wood and Ncuti Gatwa, now best known as the Doctor – had to have their chemistry reads with “other Otises”, AKA the unlucky souls who, like Martin, must have thought they were a shoo-in for the role.

As popular as Sex Education was (55 million viewed season three within a month of its release), it wasn’t exactly a Harry Potter-level juggernaut. That’s not just because of streaming’s splintering of the monoculture: the Potter films were an entirely unique phenomenon, hysterically anticipated before they’d even been made – even the open casting calls became a national obsession. But Butterfield – who was four when the first film was released – says Pottermania bypassed him entirely. “I’ve never read the books. I’m not sure why, because I loved reading when I was a kid. Maybe I was trying to be contrary.”

Instead, he got heavily into more hardcore fantasy. “Lord of the Rings was my jam. I love Dungeons and Dragons, I love my magic, my elves, my dwarves, my spells. I’m a massive nerd, and I think that stems from my love of watching Lord of the Rings in the cinema.” He goes on to wax lyrical about the movie trilogy: the motion capture, the chainmail suits worn by the Uruk-hai, the fact the cast and crew lived in New Zealand for three years. “I can quote most of the movies, and the soundtrack! Oh my God, don’t get me started on the soundtrack!”

Butterfield talks about The Lord of the Rings with misty-eyed nostalgia: contemporary fantasy just isn’t the same, he feels. “There’s The Wheel of Time and The Rings of Power, but neither of those really …” You can see the diplomacy cogs working in his brain. “How do I say this? It’s just not for me.” Would he like to have a go … ? “Yes!” he jumps in. “I would love to be in a fantasy epic!” Has he not had the opp … “No!” he cries, with mock (or maybe genuine) agony. “I haven’t had the opportunity!” Someone get this man an audition, stat.

• Second Best is at the Riverside Studios, Hammersmith, London, to 22 February.