Keira Knightley doesn't work on comedy films because she can't tell if she's doing a ''good job''.

The 'Aftermath' actress loves a ''challenge'' in her work and gets ''bored'' quickly if she doesn't feel she's being pushed, but being funny on film is a struggle for her as people on set only laugh during the first take of a scene.

She said: ''I love a good challenge. If I don't get out of my comfort zone, if a role doesn't scare me in some way, I get bred.

''I love doing great dramatic films and I'm fascinated by darker characters. Maybe that's why comedies aren't really me. The first time you say a joke, everyone laughs on the set.

''Then you repeat the line in the next few takes and nobody laughs so you haven't the faintest idea whether or not you're doing a good job.''

Keira, 34, is best known for her work on period dramas and while she used to feel ''guilty'' about making too many movies set in the past, she's now ''embraced'' that fact that's what she's drawn to.

She said: ''I love period films I always have and although I've gone through times when I felt guilty about starring in two many of them, I now embrace them. There's something about the escapism that makes me want to take on these dramas and I often relate to historical characters far more strongly than others.''

And the 'Pride and Prejudice' star has found period pieces offer her much more interesting roles.

She told Psychologies magazine: ''Sometimes those kinds of movies have been the only ones that offered me the chance to play strong, independent women. I can't tell you how many scripts I've read in which women are subjected to violence or portrayed as the passive girlfriend or wife. The period films I've done have enabled me to tell stories about formidable and fascinating women.''