Katsumi Kasahara, Associated Press
Hollywood star Lucy Liu clings onto Po, the main character of "Kung Fu Panda," in Tokyo Tuesday.

Lucy Liu had some trouble learning her lines for the hit summer animated film "Kung Fu Panda." But that wasn't her fault — a lot of the time, she said, there weren't any lines to learn.

"Working without knowing what you are doing from start to finish is very difficult," Liu said in an interview in Tokyo. "It was really improvisation. It was exciting."

Liu was in Japan on Tuesday with comedian Jack Black and other members of the film's cast and crew for Kung Fu Panda's Japan premiere.

Liu plays Master Viper in the movie, which is about an overweight panda who aspires to be a kung fu master.