1) For a book, write about what you know. For a script, write about what fascinates you. 
2) Pick the genre you're writing before you write and stick to it. 
3) Use all emotions regardless of genre. IOW, if you're writing a comedy, also have dramatic moments. It gives your script a sense of reality.
4) Economize your characters. Readers will get confused with too many people.
5) Make sure the names of your characters are different enough to not get confused. Tom and Tim = bad
6) Know what drives every character you create. 
7) Make sure your main character is in almost every scene
8) Make sure the antagonist is in the same room with the protagonist as much as possible.
9) Every scene should have some level of conflict.
10) To generate conflict, make the characters very different from one other.