The basic form of roleplaying is as follows:

Decide what your character would do and why, and then describe how they do it (stick a couple of adjectives in).

You don't technically need to "do acting"* if you don't want to, just make sure that the things your character does and the way they do them form a consistent picture of a distinct person.



* This is the obstacle for some people, the shy quiet ones, they feel like they'll be bad at the acting that people think is part of roleplaying so they'll keep quiet so nobody makes them do it. But roleplaying in a game isn't really about acting, it's about presenting a consistent character through the medium of the game.