There are plenty of ways to go about exploring and creating characters: using a questionnaire, doing exercises, or even literal role-playing. This early research and discovery process may seem overwhelming at first, but it's a necessary key to a successful screenplay. Even if you have the greatest high concept idea ever to smack Hollywood in the face, you still need to bring it to life with interesting, memorable, and – if you're doing your job right – cinematic characters.
Screenwriting is visual storytelling, so it just makes sense that you develop your characters in a cinematic way. But first figure out who your character is by understanding his or her context: his or her childhood, ethnic background, social background, environmental influences, his or her occupation, physical description, character core, his or her paradoxes, values, attitudes, flaws, emotions, and any other specific details that add to his or her depth. A character questionnaire is a great tool here, as it helps to secure the nuts and bolts of a character's foundation.
This, of course, is a great start when creating a character; however, if you truly want to create cinematic characters, you must think cinematically. Instead of simply describing who your character is, what he's been through, what his goals are, etc., visualize the character. Show him! Describe him in present tense, with movement. Make us see him. Think as if an audience is watching and learning through his actions. Imagine an actor literally acting out the character on stage as you describe him.
You don't have to limit yourself to the rules of screenwriting. Go ahead and provide "off stage" commentary: an explanation of his attitude, for example, sharing some important detail about his childhood – something we can't necessarily see. But do your best to visualize your characters in action during this crucial development stage. Remember, by doing this you're building the entire character iceberg, much of which we will never directly see in the script but is there nonetheless.