"He did play a d–head very well," Knightley tells Glamour for its July issue.
"Actually, I admire him being so willing. To play a d–head is quite a big thing when you are trying to break into film and you want to be likable," she says of PEOPLE's Sexiest Man Alive.
"With a feel-good film you are always risking going way too cheesy … but maybe I can have a little holiday where I'm not playing a character who is being destroyed," she says.
Knightley sings and plays guitar in the film, which makes her admitted stage fright seem even more surprising.
"I have really suffered from stage fright, or actually camera fright," she says. "[Because] you are presenting layers and layers of pretending, and it's not just you but other people in charge of your face or your body or your hair."
Levine has said he was also nervous while filming.
"I was worried, but I had to pretend like I wasn't," Maroon 5's lead singer told PEOPLE in April. "I kept thinking, 'I'm an actor. For this next two weeks, I am an actor.' I crossed my fingers and hoped it turned out fine."