Amanda Seyfried's singing in Les Miserables was sometimes so bad her co-stars feared she was having a stroke.

The cast all wore special earpieces to help them sing in time and on key but the forgetful actress would often leave hers somewhere on set and try to sing a capella, according to the New York Daily News.

Her co-star, British actor Eddie Redmayne, confesses that during one duet, her singing was terrible but he was too embarrassed to tell her.

He says, "I was doing a romantic duet with Amanda (Seyfried) and she was singing off-tune and slightly off-tempo, and I kept thinking, 'This is an interesting take on the song'. We got to the end and she admitted she didn't have her earpiece on. She was just singing along a cappella. I was too embarrassed to tell her."

Seyfried admits her castmates were so concerned with her odd performance they thought she was having a stroke as she attempted to keep in time without the earpiece.

She says, "I would leave it (the earpiece) somewhere and I wouldn't want anybody to know. I would just pretend I had it in. They all thought I was having some kind of stroke. It was kind of funny when we cut. They would be like, 'What's wrong with you?'"