Dedicated to going for gold

"Mental" is how James describes the physical training regime required to look the part of a sprinter. Jeanette Kwakye, the British 100-metre sprint champion who reached the final in Beijing and hopes to compete in London 2012, was her trainer initially. Shani Anderson, of 2000 Sydney Olympics and 2002 Commonwealth Games fame, then took over.

"We trained for six weeks before we started filming, which involved going to the gym five or six times a week with a trainer," says James. "We had to do weights, cardio, circuit training." Then there was the change to their eating regimes. "We ate six times a day, high protein, low everything else." Chocolate and alcohol inevitably went by the wayside. "No chance of any of that," she laughs. "It was torture."

In the film, James plays Lisa, chief rival to Lenora Crichlow's Shania. With Lisa seeming to have had every advantage in life and Shania none, the two characters are classic sporting rivals. Off camera, however, the two soon became pals.

"Lenora and me get on really well. While we were training they were trying to get a bit of competition going by showing us video clips of the other running but it didn't really work, we were just so excited for each other."

Scenes for the film were shot at the National Sports Centre at Crystal Palace and the Lee Valley Athletics Centre, both in London. The locations had the added advantage that real athletes were training there at the time, and the actors could see them in action and learn about their experiences.

Regan Hall, the director, says having athletes of the calibre of Kwayke and Anderson as trainers was invaluable. "We watched clips of the world's best athletes on YouTube to get an understanding of what a sprinter's physicality is like. As a layperson I'd never before noticed things like how a sprinter's hips move, where their knees are, how their posture is."

James and the rest of the team had to learn fast, says Hall. "Lily worked incredibly hard to bring her body into sprinter-form. On set, every time I turned around she was in the corner doing sit-ups just before going on camera and her hard work really shows on screen."

Besides the training there was the cold. Though it's meant to be a summer sports movie, the shoot took place in November. "Some days the temperatures were down to two degrees and we were sweeping ice off the racetrack," says Hall.

Having to be so focused helped, says James. "All my character in the film thinks about, all she does, is her sport. It makes her a very angry girl. I think I became a bit like that too," she laughs.

It wasn't just for Fast Girls that James had to get a move on. During the training period for the film she was also starring in Daniel Evans's acclaimed production of Othello at the Crucible in Sheffield. Appearing in the theatre every night made for long days, but it helped that waiting for her on stage were The Wire's finest – Dominic West, playing Iago, and Clarke Peters, playing Othello.

"I really didn't feel I had to act, they were just so generous and passionate that half the job was done for me by just looking at them."

James, born in Esher in 1989, has had some impressive breaks since graduating from the Guildhall School of Music and Drama two years ago. Besides Fast Girls and Othello, she had a part in the blockbuster Wrath of the Titans. There's another breakthrough part in the offing in TV's Downton Abbey, but she makes like Usain Bolt when asked to speak about the character she'll be playing.

"It's all top secret. I'm bursting at the seams to talk about it but I'm not going to."

What she particularly enjoyed about Fast Girls was the chance to take part in a film that's out of the ordinary. Too many films, she says, feature female characters who are only interested in men or love.

"The fact these girls are fierce and determined and have clear ambitions is really exciting and positive. I don't think there's enough of that about."

James, who admits she was not at all sporty before Fast Girls, says she'll take a lot from the experience, besides a renewed appreciation of the joys of chocolate and time not spent in the gym. Talking to Jeanette, Shani and other athletes, has spurred her on in her acting career.

"Nothing deters them from their goal, I have so much respect for those athletes. It's taught me a lot to live like that, about my willpower and determination and not giving up at the point of breaking.

"I really did feel so many times that I couldn't do it any more and that it was too much to handle. But you just push through. That's really rewarding and exciting to learn you are capable of doing that, we're stronger than we imagine."

Fast Girls opens in cinemas tomorrow.