When he was nine years old, maybe 10, a small, freckly, flame-haired Eddie Redmayne auditioned to be in the West End production of the Irving Berlin musical Annie Get Your Gun. Acting was a new thing for him and before the casting call he found himself dreaming – both figuratively and literally – about winning a part. He’d be in the West End! He’d get to wag school! But then, on the day, a reality dawned somewhat murkier. Around 700 children turned up. Many wore Sylvia Young Theatre School T-shirts, and danced and sang precociously behind the scenes. Each child was given a tag, walked on stage and either sang or spoke a single line. When everyone was done, a list of names was called.

The Oscars is a horse race, and I knew I was in the running, but I’d not allowed myself to believe that it could happen

“It was a meat market for children,” Redmayne recalls, wide-eyed. “It almost felt like a forerunner for The X Factor or something. So I sang my one line and was promptly sent home.” He giggles, fidgets: “I remember it being properly scarring!”

The story is 25 years old, but the memory is vivid. Redmayne, now 34, brings it up when I ask him if he expected, on the night itself, to win the 2015 Oscar for best actor for his portrayal of Stephen Hawking in The Theory of Everything. “I had let my mind fantasise before and it was cut so bluntly short,” he explains. “I’ve never actually spoken about it, but I wonder if, over years of doing auditions, I’ve stopped myself allowing to believe the dream.

“Even in the run-up to the Oscars” – he whispers those last two words like he’s faintly embarrassed to be overheard – “it’s a horse race, and I knew I was in the running, but I’d not allowed myself to believe that it could happen. And also I thought Michael Keaton was formidable and I loved that film [Birdman].”

We are in the dining room at Claridge’s in Mayfair, and Redmayne has just ordered spaghetti bolognese without looking at the menu. I have interviewed him once before, also for the Observer magazine, four years ago, and it has stayed in my memory as one of the most pleasurable assignments. It was just before Christmas, he’d clocked off for the year, so had I, and we sat in a pub for the afternoon and got a bit sloshed. He’d started seeing a new-ish girlfriend and was anxious what level of present to buy her: too showy would look over-keen, he thought, but he had once been given aftershave by a now-ex and wanted to give some hint of thought and effort. Redmayne was about to go home to his family and cook his signature Christmas Eve ham, which is so salty that everyone is dispatched to bed with a tankard of water for when they wake up, dry-tongued, in the night.

Four years on, everything is the same and everything is totally different. A north London boozer has been upgraded to a five-star hotel. Pints have been swapped for mineral water and sourdough served with “virgin” butter made by a Swedish man called the Butter Viking. Redmayne is an Oscar winner now; he was also nominated this year for another transformative performance as Lili Elbe, the first known person to undergo gender reassignment, in The Danish Girl. The girlfriend from back then, Hannah Bagshawe, is now his wife (so it sounds like he struck the right note with that Christmas present). They have a child, five-month-old Iris. Redmayne, always assiduously polite and attentive, enquires and finds my situation has changed, too – though no Oscar yet. He adopts a comic, northern accent: “We’re all grown up.”

Facebook Twitter Pinterest Feet on the ground: Eddie wears cardigan £1,130 prada.com; T-shirt £60 sunspel.com; trousers £135 hardyamies.com; trainers £370 gucci ( mrporter.com). Stylist: Hope Lawrie. Photograph: Nadav Kander/The Observer

Redmayne was no unknown when we met before: he’d just starred in My Week With Marilyn opposite Michelle Williams as Monroe; he was about to appear as Stephen Wraysford in the BBC’s impressive adaptation of Birdsong. But the stakes have been raised a little now, too. We are sitting here today ostensibly to talk about his new film, Fantastic Beasts and Where To Find Them, JK Rowling’s return to the world of Harry Potter. Redmayne takes the lead in the franchise as Newt Scamander, a wizard who, back in the day, was expelled from Hogwarts but has gone on to become the world’s foremost magizoologist (that is, an expert on nifflers, hippogriffs, bowtruckles and such exotic creatures). In this adventure, we find him in Prohibition-era New York. It is Rowling’s debut as a screenwriter, the first instalment of five, and the budget is somewhere in the region of £180m. If it’s not the biggest, most discussed release of 2016, I’ll eat my sorting hat.

So yes, things have ramped up a notch for Redmayne. Does he feel the pressure? “Well, I feel the thing has been sold as Newt Scamander at the front, but it’s actually a quartet: it’s Katherine Waterston, Alison Sudol and Dan Fogler,” he replies, in part because he’s an inveterate credit-giver and perhaps as a valiant effort to shift some of the attention. But there’s no getting away from the fact that Redmayne is the main draw here, that it’s his image, in natty bow-tie and posh-boy, mussed-up hair, on the poster. “Do you feel pressure? Of course you feel pressure. Also, particularly because I loved the Potter films. There was something so warming about being able to dive back into that world every year or two. And if you’ve enjoyed something, you don’t want to be the one who comes in and screws it up.

“But pressure’s there with every film,” he continues. “With The Theory of Everything it was knowing Stephen and Jane and the family would see the film. With The Danish Girl it was all the people that I’d met in preparation for the film who came from the trans community. It’s pressure here of a different type, which is called hardcore fandom.”

Facebook Twitter Pinterest Winning smile: receiving an Oscar last year for The Theory of Everything. Photograph: Chris Pizzello/Invision/AP

Yet, despite some categorically significant career and life events, Redmayne is keen that he at least doesn’t change too radically. And, from where I’m sitting today, he is doing an admirable job. There was a news report – scratch that: a Mail Online article – earlier this summer about how Redmayne still caught the tube. He becomes almost flustered discussing it. “The tube thing is so weird, firstly because I’ve been travelling on the tube all my life. But the whole thing I found riveting because to anyone who lives in London, how else do you get around? In a car? What, and literally take six hours from where I live to another part of London. Or you have a driver and, I don’t know, I can’t afford it! It’s a really expensive thing.”

For these press engagements, Warner Bros, which bankrolls Fantastic Beasts, hired a chauffeur, and Redmayne seems not ungrateful but compromised. “It would be quicker to hop on the Jubilee line,” he says.

The arrival of Iris in June has ensured that Redmayne’s days and nights are not especially glamorous right now, anyway. He finished work on Fantastic Beasts in May and has been fully entrenched in fatherhood ever since. There were last-ditch, child-free trips with Hannah to Paris and Japan, and most of the summer was spent at a rented house in the country, near the in-laws, punctuated with a family jaunt to Rio to watch the Olympics. Downtime is spent flipping between the odd script – he can be picky these days – and titles such as The Sensational Baby Sleep Plan. “There’s that amazing thing when you make progress,” he says. “So when Iris was sleeping through the night for the first time, you’re like, ‘This is extraordinary!’ And the next night it’s absolutely not and you feel like you’ve been pummelled.”

Facebook Twitter Pinterest Many faces: in The Danish Girl. Photograph: Agatha A. Nitecka/AP

Promotion for Fantastic Beasts marks the end of his paternity leave and Redmayne admits he’s finding it a bit disorientating. His first assignment was to introduce the franchise to thousands of rabid fans, including several already dressed as Newt Scamander, at Comic-Con in San Diego this summer. “That was so intense,” he reflects. “The whole situation is created like you are meant to go on like a rock star. I remember feeling really nervous beforehand, waiting in the wings, and [co-star] Colin Farrell was giving me a little back massage going, ‘It’s going to be fine, Eddie, it’s going to be fine.’”

When our baby slept through the night, you think, ‘This is extraordinary!’ Then she doesn’t and you feel pummelled

And here’s something else that hasn’t changed about Redmayne: near-crippling nerves have been a recurring feature of his career. To imagine that his recent success will have gone some way to alleviating them underestimates how deep-rooted they are. “I’ll always find the things that make a role complicated!” he smiles. “I was in Budapest shooting Birdsong and Tom Hooper” – who directed Redmayne in The Danish Girl and Les Misérables – “was making a commercial over there. We went for lunch one day and Birdsong was such a rigorous shoot, incredibly intense hours, jumping between time frames, the First World War, and I said to Tom, ‘This one’s the hardest. This really is.’ And Tom turned to me and said, ‘But aren’t they always the hardest?’”

Facebook Twitter Pinterest New parents: with wife Hannah. Photograph: David M. Benett/Getty Images

Redmayne clearly has a masochistic streak: armchair psychologists often connect it to a rarefied upbringing that saw him pass through Eton and Cambridge University. Whatever, he has shown himself uncommonly willing to suffer for his art: portraying Hawking – remaining slumped in his wheelchair between takes – left his spine so contorted that Redmayne’s osteopath wanted to use him as a case study; The Danish Girl involved meticulous, often minute changes in speech and movement to project accurately the transformation of the Danish artist Einar Wegener to Lili.

Please tell me, I say, that making Fantastic Beasts was more straightforward? Redmayne winces. “For this, it was working with a lot of invisible creatures that weren’t there and trying to find a way to negotiate that. And Theory and The Danish Girl were both eight-week shoots and this was a six-month shoot, so it’s the difference between a sprint and a marathon. So there were moments on this where I was like, ‘No, no, no, this is the hardest.’ I think it’s probably just human nature.”

Facebook Twitter Pinterest Dark arts: with Katherine Waterston, Alison Sudol and Dan Folger in Fantastic Beasts. Photograph: Jaap Buitendijk/AP

There are two activities, Redmayne has learned, that can reliably silence the noise: drawing and playing the piano. “Particularly at the moment they fill quite a large part of my life,” he says, and he has a grand piano in his flat in Bermondsey, south London. “As far as getting out of your head, when you’re playing the piano your focus is so all-consumed by trying to get that flipping note right that you can’t think of anything else. Similarly when you’re drawing something, the focus between what’s there and the paper and what it is you’re trying to recreate – everything is the opposite of freed. It’s very much restrained, but sometimes I find that quite calming.”

Facebook Twitter Pinterest Eddie wear cashmere knit £360 paulsmith.co.uk; white t-shirt £60 sunspel.com Photograph: Nadav Kander/The Observer

A woman from Warner Bros materialises to tell us our time is up. Redmayne bolts a last mouthful of spaghetti and profusely apologises: “Sorry we couldn’t get drunk like last time.” He can’t say how many of the Fantastic Beasts films he’s signed on to appear in, but it’s safe to assume that this could be his world for much of the next decade. In between, he will continue making life as hard as possible for himself with challenging roles, obsessively prepared for.

As a parting question, I ask Redmayne what he enjoys about acting. It sounds so torturous how he describes it. He goes silent for a few seconds, before recalling a single performance of the play Red that he appeared in from 2009 to 2010. There was a scene at the end where the painter Mark Rothko (Alfred Molina) fires his assistant Ken (Redmayne) and Ken thanks him. For the entire run – for which Redmayne won an Olivier award in the West End, and a Tony on Broadway – he played it one way, but in the final performance in New York, he changed the intonation of the line so that he was expressing gratitude to Molina instead.

“Something in the way I said ‘thank you’, I knew from me it was unfettered with any bullshit,” he says. “It felt true and what was interesting was you feel the audience move in a little bit. And in a second it was gone.” He clicks his fingers, “But those are the moments you keep doing it for.”

Fantastic Beasts and Where to Find Them is in cinemas from 18 November

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