Most good movies operate on two levels. These two levels derive from two elements of the main character: Want and Need.
The main character’s want drives the story. Sometimes this want comes out of the character themselves – in Almost Famous William wants to be a rock journalist; in Little Miss Sunshine Olive wants to be a beauty queen; in Star Wars Luke wants to adventure in space. They start the story with these desires.
Other times the want is thrust on the character by an event in the story – in The Fugitive Dr. Kimble wants to prove his innocence after he has been wrongly convicted of a crime; in Some Like It Hot Joe and Gerald want to escape mobsters who are trying to kill them because they’ve witnessed a murder; in Back to the Future Marty wants to get back to the future because he’s been accidentally stranded in the past while fleeing bad guys. In these cases the event that creates the want is usually the “catalyst” beat of the three act structure.
We all want something every waking moment, even if it’s just to be left alone or to take a nap. Characters may have different wants that drive them through each scene, but there will always be an overall want that drives them through the story.
In most (but not all) good movies the character also has a need which is different than the want. They are usually unaware of this need. It is a change the character has to make in order to be happy. The need drives the character arc and adds depth to the story. In the best cases the need is tied to the want practically and thematically.
For example, in Almost Famous William needs to learn to be "honest and unmerciful" in order to achieve his want of becoming a good journalist. In Star Wars Luke needs to learn discipline, patience and faith to destroy the Death Star and become a hero. In Back to the Future Marty needs to gain self-confidence which he does by teaching that same lesson to his father thus undoing the damage he’s done to history and preventing himself from never being born (which all makes perfect sense if you’ve seen the movie!)
Sometimes a character’s need is to give up what they want because that want is preventing them from being happy. Consider Casablanca: Rick’s want is to be left alone – not get involved. He doesn’t stick his neck out for anyone. This is established from the beginning of the film. His need is the opposite – to take a stand, restore his passion for lost causes. Until he does he will not be truly happy. The arrival of Ilsa is the catalyst that challenges his want and causes him to undergo a transformation which ultimately leads him to take a stand and become happy again.
Structurally, the want and need generally drive two parallel stories, one internal and one external, which interact through the course of the movie.
The external story is built on the character’s want. This is the visible, “physical” action of the movie. We watch the character actively pursue their conscious goal.
The internal story is built on the character’s need. This is the story that’s happening in the character’s head/heart/soul as the character transforms to achieve (or perhaps not) their need. Our job as writers is to find ways to externalize this journey so the audience can see it.
Identifying your character’s want and need and then tracking the internal and external storylines will help ensure your stories and characters are rich and deep.