(Cover) - EN Showbiz - Keira Knightley says she gets a kick from watching her co-stars cry.

The 25-year-old British actress has appeared alongside some of the most talented leading ladies in the business, including her good pal Carey Mulligan in 2005s Pride & Prejudice and new movie Never Let Me Go, and close friend Sienna Miller in 2008 film The Edge of Love.

Keira enjoys working with both stars, and admits shes particularly impressed by their ability to cry on screen.

God, she's a brilliant crier! Really. She's on another level, Keira said of Carey in Never Let Me Go.

That was the scene where I said to her: Right, every time I say this word, I want one teardrop to drop out of your right eye. And it was like clockwork. I've never seen anything like it. Actually, no. Sienna Miller can do the same thing. It's astonishing. I love it. I got such a kick out of it.

The star admits it can be tough watching her friends shedding tears - and she especially struggled portraying manipulative Ruth in Never Let Me Go, where Carey plays kind-hearted Kathy. Based on the novel by acclaimed writer Kazuo Ishiguro, the movie recounts a love triangle which Carey and Keiras characters are involved in. Keira felt she had to apologise to Carey for her character Ruths nasty behaviour whenever filming ended for the day.

It's horrible, isn't it? I had to say to her after every scene, I'm just so sorry. She'd say, It's all right. It's pretend, Keira told guardian.co.uk. (C) Cover Media