WHEN the call came, Jeremy Irvine was at home building a go-kart with one of his younger brothers.

Even then it didn't seem like the biggest deal in the world. This would be his sixth audition for Steven Spielberg's War Horse, and Irvine, who had only had one job previously, playing a tree, looked on this one like the rest – as good experience, no more.

The scene he was told he had to read was between Joey, the war horse of the title, and his young owner, Albert. Don't turn over the script till I say action, said his agent. When he did, the first line was: "Joey, Joey, Steven Spielberg wants me to play Albert in War Horse."

Irvine laughs at how his agent mocked up a script to spring the surprise. "That's how I found out." It wasn't until the champagne cork popped that it really sank in. Fresh from the film's royal premiere, and having taken his grandma to Buckingham Palace the night before for a post-premiere reception, Irvine is still fizzing with the excitement of it all.

Spielberg, in London this week for the premiere, said he had looked at "hundreds" of potential Alberts. "Nobody had the heart or the spirit or the communication skills that Jeremy had."

The fact that he was a newcomer wasn't a problem for the director who had given Drew Barrymore and Christian Bale their first major roles, in ET and Empire of the Sun respectively. Indeed, Irvine's lack of experience was regarded as an asset.

"I really trust the authenticity of real people," said Spielberg. "My job is to get them to be themselves in front of the camera. Often what happens is you get a newcomer in front of the camera and they freeze up, or they imitate other actors or other performances they've admired and they stop becoming themselves.

"My job as a director is to return them to what I first saw in them, which is simply an uncensored human being. I didn't want Jeremy to be a character actor, to be somebody he wasn't, I simply wanted him to be the person he is today and he did a wonderful job playing himself."

When I meet Irvine, "himself" – the son of an engineer dad and a mum who is in local politics – is playing the part of a guest of Claridge's. It's the lunchtime after the premiere the night before. Has he ever stayed here before? "Course I haven't," he laughs. "It's a nice little step up."

For "nice" and "little" read Everest-sized. Lest you think Irvine, born in Cambridgeshire in 1990, is simply the luckiest young actor alive, it should be pointed out that his progress from unknown to Spielberg movie has also involved some slog.

After drama school, and wanting to "get away from the crowd a bit", he set out to find an agent himself. He made a showreel with a cameraman pal and "walked around London for two years, putting CVs through letterboxes". His first job, his only job before War Horse, was as a tree in an RSC production. Still, it got him an agent and that agent put him up for War Horse.

Meeting Spielberg for the first time was the first point on a learning curve that also included being taught how to ride. The first task was made easier, says Irvine, by Spielberg's ability to put actors at their ease.

"He's so generous with his time, really talks to you. There's no rehearsal, so what you see on screen is probably the first or second time you've ever done that. He somehow makes that experience, which for an actor should be terrifying, very relaxed."

Then came the horses. Eleven horses played Joey at various stages, with Irvine having to get to know them all. "I wouldn't have called myself an animal person, I thought I was going to have to act it. But within a week of being round the horses I was a sucker for them like everyone else."

Their size and strength must have been scary for a novice, I suggest. "I didn't have time to be scared. That wasn't really a choice. There was no time for mistakes, I just got on with it."

Irvine comes across as a get-on-with-it type of person, which must have reassured Spielberg. He wanted a newcomer, but he also needed someone who could cope with the pressure of a huge production, and handle the hoopla after. Irvine has had more of a chance than most to show his maturity. Diagnosed with Type 1 diabetes at the age of six, he had to learn early on how to manage and live with his condition.

The self-discipline that must have involved showed when it came to War Horse and working with horses. His co-star, Tom Hiddleston, who underwent equine training with Irvine and others in the cast, remembers the youngster being the first to arrive at the stables in the morning, and the last to leave.

Irvine says: "I was astonished at how quickly I built genuine relationships with these animals. I'd spend all day every day for two months with all the Joeys, cleaning them up, mucking them out, really living with them. By the time we finished they were not just relationships but strong relationships."

Irvine might not have thought of himself as much of an animal person before, but the trait runs in his family. His other younger brother is studying to be a vet. One of his great grandfathers, both of whom served in the first world war, had a war horse of his own. She was called Elizabeth, and, like Joey, she had a white cross on her nose and four white socks. His great grandfather bought Elizabeth after the war to save her from being made into horse meat. Irvine has seen the receipt.

When he won the War Horse part the casting director told him it would lead to other work and to choose wisely. Besides parts in the dramas The Railway Man, in which he plays a young Eric Lomax (Colin Firth is the older Lomax) and Now is Good, he will be seen later this year in Great Expectations, directed by Mike Newell.

Yet another version of Dickens's tale? This one will be worth waiting for, says Irvine, who plays Pip. "David Nicholls [the author of One Day] has written it. When he takes on a script you know it's going to be something different. He hasn't written a 'Dickens' film, he's written a thriller. It's shot in a very film noir, Third Man sort of style, it's dark and it's gritty."

This time his co-stars will be of the Ralph Fiennes and Helena Bonham Carter variety. Not many horses, or at least none to compare with Joey. Irvine knows he's been lucky, but he also remembers that casting director's advice about working with people he can learn from.

"That's what I've done so far," he says. "And it has been quite a ride."

War Horse opens in cinemas tomorrow.