'He's really good, he can do it all, he's one of those...' gushed the 25-year-old blonde, who had to kiss Timberlake, 30, during filming.
'He's also really funny. I'm kind of sad that we are done shooting this movie,' she said.
'He would constantly break out into song which is kind of special, because he has that perfect voice. Even though he's mocking himself he's great,' she added.
Amanda appeared on US talk show Regis and Kelly to talk about her new thriller film Red Riding Hood.
The young actress also spoke about her joy at seeing Colin Firth win the Oscar for Best Actor for The King's Speech.
'I have never seen him so happy in his life,' said the star, who worked with the 50-year-old British actor on Mamma Mia.