How much is revealed about the storyline and characters in a film opening?

The opening of a film, in my opinion, should simply make the audience want to watch the rest of the film.

To do this we should meet the main character(s) and understand their role in the film. It doesn't matter how much information we are told about the main characters as they are the basis of the actual film.

Alternatively, I think we should find out little about the storyline itself. There may be a few hints of what the storyline entails but I think that the characters entice the audience to continue watching. Plus, the storyline is needed for the main sequence of the film.

This views can change dependant on the genre of film. For example, horror films may not introduce main characters at the beginning, instead they'd try to produce a certain tension.