the princess Diaries (2001)

Fat Louie.
A geeky girl finds out she is not the loser she thinks she is, but a European Princess, who happens to own a cat. The cat appears throughout the film, doing cat things - mainly sitting and watching the action.

cat in the cage (1978)

Sampson.
A man returns from hospital, to find his cat there to welcome him. He's told that the cat scratched his new step-mother, and was given away as a result. But he found his way back home and he and his master spend the film tormenting this woman.