IN a way, Harry Styles has always been a film star. He just hadn't been in any movies yet.

That charisma, that hair, that nagging feeling he was always going to outgrow the boyband that made him famous, were all the hints we needed that he was a Hollywood star who hadn't made it to Hollywood just yet.

That has all changed with Dunkirk, Christopher Nolan's relentless, spectacular Second World War epic about the evacuation of British troops from the French beach, where they were surrounded by enemy forces.

While household names such as Sir Kenneth Branagh, Sir Mark Rylance, Tom Hardy and Cillian Murphy headline the cast, Styles is part of an ensemble of mainly unknown actors who portray the painfully young soldiers fighting to survive.

Now 23, he has been famous since he first auditioned on The X Factor when he was 16, a move that landed him a spot in One Direction and changed his life forever.

But that wouldn't be the last audition he would do, Years later, with the band on hiatus, Styles was back in front of appraising eyes to see if he had what it takes to impress Nolan.

And Nolan was duly impressed, saying: "When you've found the right person for the role, as I had with Harry, you have to follow that.

"Whatever baggage there is, whatever, it's not different to casting Ken Branagh or Mark Rylance and worrying about what they have done before or Cillian Murphy in Peaky Blinders.

"With great actors, and Harry is a great actor as people will see when they see the film, you don't worry about other things they have done, you trust that they are going to create a characterisation for the audience that allows them to lose themselves in the audience.

"That is exactly what Harry does in the film and I was confident that he would be able to do that."

Styles actually did one of those first auditions with Fionn Whitehead, the young star of the film whose face is on the posters but whose resume only had three episodes of a TV mini-series on it before he landed the role of a lifetime.

Sitting together in a London hotel room, Styles recalls: "We auditioned together and I was sitting in a cold lonely room and Fionn came in from a set of something he was working on and there was one other person in there.

"You had no idea what part you were auditioning for, we didn't know how many parts there were and they kind of round-robined us a little bit, did everyone with everyone."

After months of shooting together, often underwater, the young men are clearly very comfortable together but Styles is the more gregarious of the two, bounding out of his seat to proffer kisses on each cheek.

"Thanks for having us," he says in his Cheshire accent. "Thanks for coming. Where have you travelled from?"

This disarming charm will not be surprising to the legions of fans who have followed Styles' every move for years, obsessed over his romantic entanglements and propelled his solo music to the top of the charts.

But for someone who has achieved so much so young, he says it was "overwhelming" when he first arrived on the Dunkirk set, on the actual beach where the evacuation took place in 1940.

He adds: "I think as much as you can imagine what something like that is going to look like, people don't make that. I don't know in what other situation someone gets to do what he (Nolan) did so he was pretty amazing to be around.

"You are kind of in awe of it a lot of the time, just continuously during the film.

"At times the tide would go out and you could see ruins of some of the old boats that were out there and it was moments like that when everyone was aware of how special the place that we were was."

Whitehead adds: "On reflection, the most incredible thing about the set, and there were loads of things, like the Spitfires flying overhead, was the fact that they recreated the mole (a stone breakwater pier).

"It's been ruined by bombings and weather and the fact they went and built it, half a kilometre out into the sea, it was insane. It was a crazy thing to be a part of."

And for Styles, the time on set gave him the chance to get an acting bootcamp from the best British actors working today.

"Any time you're around people you're a fan of, you just kind of try to soak up as much of that as possible and watching them work is a privilege.

"So many amazing people worked on the film from the cast and the crew and you just loved being around them, and getting to watch them work, it was pretty amazing."

He was particularly bowled over to be making his acting debut in a Nolan film, having loved the director since his 2000 film Memento, starring Guy Pearce.

"I think I always found his structure so interesting and in terms of the way he keeps stuff from the audience when the characters don't know about it and it hits you so much harder.

"You always feel like you're right alongside the character, rather than watching a film of stuff happening to them."

So with one film under his belt, the world is waiting to see if there are any more to come.

He is about to embark on an extensive tour to support his self-titled debut solo album so if he does act again, it won't be for a while.

But right now, he says: "I might be one and done, I think I'm one and done to be honest. I think so."

Oh Harry, say it isn't so.

*Dunkirk is in local cinemas now.