London: Actor Keira Knightley says it is difficult to judge an actor's performance in comedy films and that is the reason why she refrains from doing them.
The 34-year-old actor is particularly associated with period dramas, having starred in films such as "Pride & Prejudice", "Atonement" and "The Duchess", among others.
In an interview with Psychologies magazine, Knightley said she looks for those parts which challenge her as a performer.
"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," she said.
"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," she added.
Knightley most recently featured in "The Aftermath", another period drama. The film, which also starred Alexander Skarsgard and Jason Clarke, released in February this year.