A successful character has to be memorable, and something that the audience can relate to or at least understand. Film makers only have a few minutes to sear the characters into the audience's mind, so that in itself takes some skill. Its important to give your characters strong personalities, wants, and goals, so the audience will like them and will want to follow their story throughout the film.
Indiana Jones is a good example of this, because he is a well rounded character. He is an archaeologist and explorer, but is also a college professor back at home. His journey is inspired by selfish reasons as well as influenced, and tweaked by his few morals. He makes the audience want to become an archaeologist just so they might be able to have a life remotely like that of Indiana Jones's.
The most important part of the movie is arguably the characters, because even with all of the action, adventure, drama in the world, movies are more about the characters that these events happen to.