Carey Mulligan is brilliant at crying: Keira Knightley

London: Hollywood beauty Keira Knightley says her `Never Let Me Go` co-star Carey Mulligan is brilliant when it comes to crying onscreen.

The 25-year-old actress said that Mulligan, who plays Kathy in the movie adaptation of Kazuo Ishiguro`s dystopian novel, was `on another level,` a daily reported.

"It was like clockwork. I`ve never seen anything like it," Knightley said. "Actually, no. Sienna Miller can do the same thing. It`s astonishing. I had to say to [Mulligan] after every scene `I`m just so sorry.` She`d say `It`s all right. It`s pretend,`" Knightley said.

The `Pirates of the Caribbean` star was elated to reunite with Mulligan after playing sisters in Jane Austen`s adaptation of `Pride and Prejudice`.

"One of the big reasons I wanted to do this was the opportunity to work with a friend because it happens so rarely," she added.