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.