Aged 23 and barely two years out of drama school, Lily James is already a veteran multi-tasker. She was playing Desdemona opposite Clarke Peters and Dominic West in Sheffield last year when she landed her first lead film role, in Fast Girls. By day she would pound around the track at Don Valley, getting into shape to play a British sprinter. By night she would squeeze into period costume for Othello at The Crucible. "I was wearing a corset and I wouldn't be able to breathe because my abs were hurting from all the sit-ups I had to do."
Aching muscles notwithstanding, her tragic heroine drew "star is born" notices. "She practically sweeps all before her as Desdemona", rhapsodised The Daily Mail. "Poise, diction, allure – she has them all". There was no time for resting on laurels, though. It was straight into six weeks of intensive preparation for Fast Girls. The film, an unashamedly feel-good Olympics release, follows a sprint relay team on their way to the world championships. Produced by Damian Jones (The Iron Lady) and written by Noel Clarke, Jay Basu and the playwright Roy Williams, its female-led cast, slick urban style and pumping soundtrack make it the quintessential 2012 movie – Chariots of Fire crossed with Bend it like Beckham.
Just as the latter made a star of Keira Knightley, Fast Girls is set to do the same for James, who plays Lisa, an ultra-competitive spoiled brat with a swishy ponytail and impossibly flat abs. Hollywood has already taken note; she notched up her first blockbuster, Wrath of the Titans, in March.
For Fast Girls, she trained five times a week with the British athletics team – running up hills until she vomited, clocking up 800 sit-ups a day and eating a high protein, low-everything else diet. "I've never gone to such physical limits," she says. On set, she began to blur into Lisa. Director Regan Hall recalls how every time he called "Cut!"she would drop to the floor to do crunches. "I became obsessed", admits James. "I lost almost a stone. It's amazing to see how quickly your body changes. It was empowering."
Still, she was glad to leave the character behind. "There are other things I want to focus on rather than staying in shape. It scares me a bit, the side of me that gets so focused on one thing. You can lose sight of reality."
This week, she's back in a corset, shooting Downton Abbey. "I come in at the end and I'm playing a posh person. Surprise, surprise..." While keen not to be typecast – she immediately dyed her hair back from Lisa's sleek blonde to her natural "dreadlocky" brunette – her roles have had a hint of English rose so far. On graduating from Guildhall in 2010, her first job was playing big sister Ethel in Just William followed by Poppy, flatmate to Billie Piper's Belle ("not a prostitute!") in Secret Diary of a Call Girl.
Born into a theatrical family in Esher, Surrey – her grandmother, Helen Horton, was an actress and her father, who died of cancer in 2000, also acted – her first love was musicals. As a student, she starred in The Last Five Years at the Barbican with her classmate Freddie Fox and has since appeared on stage in Vernon God Little and Martin Crimp's Play House. "I want to experience everything. I saw Cate Blanchett in Big and Small and it was mindblowing. The fact that she can do theatre and is also a huge movie star is really exciting," she says. For now, Desdemona remains her high point. "I can't believe I got to do that. Clarke's passion blew me away. He made it easy. He'd stand in front of me with tears in his eyes and I didn't have to act anymore."
Fast Girls is released on 15 June