It's your job as a writer to create characters in a screenplay. Memorable characters are flawed yet have strong wants that propel the action of the story. Memorable characters that come to mind are Alvy Singer, Michael Corleone, Max Fischer, John Ferguson, Tony Soprano amongst a few. As a writer you imagine the character and put him on the stage of your screenplay. The biggest challenge is that once you've created the character you need to let them act consistently. In all my years of reading aspiring writers' scripts, I've noticed that the biggest script flaws are rooted in the writer dictating the characters actions and not the character dictating actions. When you read writer-dictated characters, the story feels false. It's hard to feel empathy when a character isn't acting as we think they should. This is why I've found working with good actors in script development can be very useful. They immediately catch when the character is being false or untrue to the character you've created true nature. I feel like as a writer you always want to see your character in some "cool" situation that will be visually amazing. But this forces the character to act in an untrue way. Writing from the characters perspective is incredibly hard and a talent that I think determines writing success. It's like when an actor feels they need to add some flare to words to be interesting. We watch that and it feels fake. Just being real and in the moment is so entertaining. It's hard to accept that and do it.
When writing a script, it's important that the character arc over one personal issue in the story -- this can be a value or institution like marriage, adulthood, fatherhood, or commitment. It's easy to start with a character who has issues with commitment and then end it with him getting over issues of pride. What you decide as the outcome and how this character evolves over this one issue, is the theme of your story. Why does it have to be one issue? That's what's satisfying to the audience. It came to me while taking my wellness walk around the Historic Filipinotown where I work. I thought, "Here I am walking up a hill and it's costing me lots of energy. That payoff is easier when I get to coast and walk down hill. That's my payoff." If I were to walk up this hill and then time warp -- this is what bad screenwriting does -- to another part of town and continue walking uphill, I wouldn't have earned the payoff for my audience. Hmm, but if I did go to another part of town and I did end up at a downhill or even flat place, wouldn't it be a payoff. Perhaps the metaphor isn't holding up.
The idea here is that an audience feels most satisfaction when it has earned the catharsis. So if a character is dealing with an issue of commitment and arcs on that, the audience will feel satisfied. If however, the character arcs on pride, they haven't earned the change or seen the character deal with pride and so it's not satisfying. The reason this is easily done, if that if you focus on writing to the external goal of the character, it's easy to forget the internal. That is, they might still be going after the same external goal throughout the movie, but arced or changed for different internal reasons. This is when the character arc isn't satisfying to the audience.