Keira Knightley says she signed up to new thriller Jack Ryan: Shadow Recruit because she was tired of playing tragic heroines in her movies.

The 28-year-old beauty stars opposite Star Trek star Chris Pine in the action flick which is based on novelist Tom Clancy’s CIA agent hero and directed by Kenneth Branagh.

Speaking at a press conference in London today, Knightley said: “I think when I finished doing Anna Karenina I realised that I’d been playing characters that pretty much died or something horrendous happened to them for about the last five years and I fancied doing something fun.

“I’ve never done a thriller before, and it’s been a genre that I’ve really liked. It’s a difficult one because you combine action with proper storytelling and it needs a proper storyteller to do it, so I was excited by the challenge.”

When asked what attracted her to the role. she said: "As far as the relationship between Jack and Cathy goes I was interested with the idea of what happens to a relationship when there is a secret, when there is something that can’t be discussed, something that can tear two people apart even though they are completely in love.

“I thought that was a very interesting part of the story, as far as what people who work in secret services have to give up and the emotional toll that it takes on them.”

Jack Ryan: Shadow Recruit is released in cinemas on Friday.