Close-up: Naomie Harris

Keanu is cool, says the actress – but he's not a patch on London

It's been six years since Naomie Harris grappled with the living dead in Danny Boyle's 28 Days Later, and in that time she's appeared in the likes of Miami Vice, A Cock and Bull Story and not one, but two Pirates of the Caribbean films. But now the 31-year-old Londoner is returning to the small screen to play the eponymous mental patient in Poppy Shakespeare, a feature-length adaptation of Clare Allan's novel.

"I like to do completely different characters," says Harris, "and Poppy was unlike any character I've played before." Indeed, Poppy is not your average mental patient. In fact, she believes herself to be entirely sane – unlike her friend N (Anna Maxwell Martin), whose ambition is never to be discharged from the hospital. "That's what the whole piece is playing with," ponders Harris. "How much does it take to push someone over the edge? Everyone is teetering on the edge, especially in a fast-paced city such as London."

Unsurprisingly, the actress found it difficult to slip into Poppy's shoes every day. "It left me feeling emotionally raw," admits Harris. "Anna really helped, because she's so much fun to be around. It's such a bleak subject, so you need someone who's light and fun on set."

After her spell on the small screen, she's heading back to Los Angeles for her next round of auditions. She's not bad at them, either: in her next Hollywood blockbuster-to-be, Street Kings (out in April), you can catch Harris opposite Keanu Reeves. But despite her Hollywood sojourns, Harris is a Londoner at heart: a recent experiment in country living was cut short when she started to miss her social life. "I had this amazing farm – it was so beautiful. But I just thought, I'm too young to be stuck out here with the sheep!"

'Poppy Shakespeare' is on Channel 4 tomorrow at 9pm