When it comes to creating a character that truly stands out – one that jumps off the page and sticks with the reader like a jingle that takes up a permanent residence in your head – it's almost always because the writer knows the character intimately well. The writer knows the entire iceberg.
Conversely, when the writer forces a character to do or say something simply because the writer needs that character to fulfill a particular plot requirement, the execution of the scene will inevitably read as forced and contrived. However, if the writer knows the character down to every last detail of his or her core, when it's time for the character to take action or deliver a line of dialogue, it's the character who writes for the writer – never the other way around.
But how is this accomplished? Research. Lots and lots of research.
Now when it comes to one-sting characters, minor supporting roles, or High Concept story driven films, character research may be less important, but if we're talking about creating a memorable protagonist in a character driven screenplay, research is definitely a crucial asset.
First, you must explore your character, using a character questionnaire to discover and develop as much as possible. You should know your character's surrounding environment as well as any influences: ethnic, social, religious, or educational. It's also helpful to understand how the historical context, location, or occupation might impact your character.
Moreover, you need to have a clear visual impression of the character's physical description as well as a solid understanding of his or her psychological core. It's essential to define your character's consistencies and paradoxes, understanding his or her values, attitudes, and emotions. And you should definitely create a detailed backstory, understand your character's psychology as well as his or her past and present relationships.
I know that's a lot, but believe me – you do all that foundation work and when it comes time to pounding out script pages, you'll be in for an easier, much smoother ride.