The prose writer has freedom to use anything, go anywhere, use any tense, and explore any point of view. Screenwriting, however, is essentially filmmaking on paper. It’s a visual medium after all, so the screenwriter must write in PRESENT TENSE – only what the audience can SEE and HEAR.
Imagine sitting in the theater yourself and seeing your words as the reader would. If you write a scene describing what a character thinks as she stares pensively out a window, all we can see is a character staring out a window. Nothing more. We can’t read her mind; we don’t know her inner thoughts. So what’s the fix?
Bring in another character that sets off a verbal argument, or if it’s pivotal she’s alone, use the environment around her (pictures, documents, props, text messages, etc.) to reveal her thoughts. Now it’s the discovery and her reactions to these objects that move the story forward.