I think as a screenplay writer you should try to create believable characters.
But beginners (like me) often write characters who can do everything, are famous, and popular. Why you just don’t give them a sign where be written on it: “Hey, I’m a Mary Sue. Hate me!”
For example, I read a story about an outsider who gonna be quickly famous and popular in his school after 5 paragraphs. Seriously? You wanted to write a story about an outsider. Not a clichéd High school story.
So, how do I avoid this when I already planned my character? You know they shouldn’t gonna be like that while I write the script.
Please, give your best tips & tricks to avoid a Mary/Gary Sue while writing a screenplay.