Envision a person in your mind and write a basic description of how she looks, talks and dresses. If it helps, you can imagine yourself interviewing imaginary characters for a role in your screenplay. Imagine a room and write down a brief description of it, then let a character walk in and start doing something. Talk to the character, ask questions, suggest activities. Write how the character responds, looks and acts, doing your best to write quickly and censor nothing. Repeat this exercise until you've seen a few potential characters.
Choose the character that appeals to you the most and seems most fit for the role. Brainstorm and write down what that character wants and why. The characters' conflicting desires will drive the whole story, so if you can't find a desperate desire that the character is willing to act on, keep trying until you find a character with a driving desire.
Begin to write down your ideas of who the character is, including her appearance, personality, career, tastes, fears, mannerisms, friends, family, notable moments in her life and the motivations that drive her. Most of this will never appear in the screenplay, but it will add the ring of truth by helping you write the character as if you know her.
Write short scenes with your character in different unusual situations. These are not to go in the story but to help you think through what drives the character and how he responds to unexpected and difficult events.