Determine your character's goal. Your central character has a conscious goal which he outwardly strives for, but the story's opposition makes this goal nearly impossible to reach. The opposition is usually a person with the same goal or for some reason opposes the central character's goal.
Happiness is not a goal, but seeking $50 million in a duffel bag somewhere in the desert is. Winning a dance competition is a goal. In "Romancing The Stone", Kathleen Turner's goal is to find the stone so that she could rescue her sister. Her goal is opposed by Zolo and the kidnappers who all want it as well. To top it all off, Michael Douglas wants it as well.
Determine your character's motivation. Your character has to be motivated in order to achieve his goal. Ask yourself why does your character want what he wants? The more personal the motivation, the better it will be.
Rocky's goal in the first "Rocky" film is to go the distance with the champ (Apollo Creed). Why does he want to do this? So he can prove that he's not a bum. In "Rain Man", Tom Cruise's harsh treatment from his father is his motivation behind collecting the inheritance. He wants to get even with his father.
Determine your character's inner need. Beneath the character's conscious goal is an unconscious need that has to do with self-image--desiring a better life or seeking romance for instance. This yearning to be truly fulfilled and happy may run counter to the character's conscious goal or can support it.
In "Twins", Danny DeVito's character is after the $5 million (goal) that a really evil bad guy is also after. His character also yearns to be loved by a family (need) but this is blocked by his selfishness and greed.
Determine your character's flaw. The character's need is blocked by a flaw which serves as a form of inner opposition working against her inner need. The flaw can be a form of pride, selfishness or greed.
This flaw usually comes from the character's backstory. Something happened in the character's past (most likely before the movie begins) that hurt or changed her so she lives and acts in inappropriate or hurtful ways.
In "Jerry Maguire", the character has two flaws and growth arcs. His flaw in the Action side of the story is self-doubt. His flaw in the Emotional/Love side of the story is his inability to love and be intimate.
Write your character's backstory. Something happened to your character before the first page of your screenplay that serves as the reason for why he is the way he is.
In "Sleepless in Seattle", the backstory revolves around the death of Tom Hanks' wife. In "The English Patient", Hana's backstory is driven by the belief that everyone she loves dies.
List your character's personality traits. It is important to identify your character's most dominant personality traits. In "Snow White and the Seven Dwarfs", the dwarfs were nothing but seven old guys who acted and looked the same--until Walt Disney gave each of them a human trait they could be identified by.
Give your characters no fewer than two or three dominant traits to begin with and work your way up from there.
List your character's skills, knowledge and props. Ask yourself what your character knows how to do. Maybe he used to be a part of a war. Maybe she used to be a locksmith. The possibilities are endless.
In "My Cousin Vinny", Vinny's fiance is an unemployed hair dresser who knows everything there is to know about cars because her family has a strong background in automobiles.
Does you character have any props that they may carry with him? A cross or a picture of a loved one? It could even be a tattoo. It has to be something of importance to him.
List your character's quirks and imperfections. Consider problems and imperfections that are familiar to all human beings. Is your character a grouch? Maybe your character doesn't like dealing with people until she's had that morning cup of coffee.
In "Saving Private Ryan", Tom Hanks' character has Parkinson's disease. It wasn't part of the story, but it was an imperfection of his character. In "Star Wars", Darth Vader has plenty of imperfections.