So - here's a fun screenwriting challenge / exercise for you:
Pick your favorite TV show and write a dramatic scene between its characters. It could be an argument, a comedic moment, or an offshoot of a similar scene. Make sure the characters in your script are aligned with the characters in that show. If you're a big fan - you'll know how these characters can and will react to certain situations, how they'll speak, and what they'll say when pushed against the wall. In other worse - you know the characters.
Once you have that, change the characters names and voila! You have a short film with three dimensional characters. The purpose of this exercise is to show you how well you can write when you understand the characters you are writing for.