Question: How do I develop a character quickly without giving anything away or tell their whole life story. I need to be able to give enough insight into my character so the first scene would make sense on how they act. How do I develop a character within two or three pages?
Sometimes you just show them in action, let the reader see who they are, what they can do, how they behave. Later on in the story, you can show or explain what made them the person they are.
Readers don't mind a little mystery in the opening pages. In fact, it can be a good thing. It keeps them reading in hope of getting their questions answered.
If we are talking about the main character, it helps to show not only his/her unique abilities in the opening pages but also his/her achilles heal - some weakness that can also be a mystery, for a while.
Think about the opening sequence of the first Indiana Jones film. It's only a short space of time. But by the time Indiana escapes on the plane, we know a lot about him. We know he is skilled with booby traps, a whip, etc. We know about his expertise, his rivalries, his purposes (preserving artifacts for museums). We know how he handles challenges. We know his values (loyalty). And we know his biggest fear (snakes).
However, this sequence also leaves us with a few mysteries as well. That's okay. His background and other aspects of his character are revealed as the film progresses. And some key facts about what made him the man he is are not revealed until the third film.