Next to 'write what you know' SHOW don't TELL is one of the oldest pieces of advice in screenwriting; and rightly so! A Screenplay is the text basis for what will become a visual final product. It's that eventual translation to film that makes screenwriting so tricky and yet so interesting too. IF it were only going to remain as words - it would be a novel or short story.
So the first thing you need to do is figure out what your character is feeling in every single scene that you write. And then you need to figure out an action - that is true to your character - that SHOWS or demonstrates or telegraphs that feeling. This can sometimes be hard! Let's try one simple, basic emotion as an example and see what happens.
Anger is one of our most basic human emotions, but everyone presents or displays their anger in different ways. One character might blow up, lose their cool, or even clobber someone in anger. This is very straightforward mechanism for SHOWING a character's anger response in a situation.
Now, what if instead of blowing up when provoked to anger - you create a character that cries as a response. How does that change our understanding of WHO your character is? Does it make your character more childlike in our eyes; or more female perhaps? Does it change our understanding of the quality or depth of their anger?
Are you starting to get the picture as a writer. The basic emotion your character feels in every scene is conveyed to the audience through an action. That is how we read what is going on inside your characters heart & mind. You can radically influence our perception of your character by choosing an action different than what's expected or normal as a response.
And then, to add to the mix.... an actor will read the part you've written and add their own interpretation to it. Perhaps the actor decides that the character then gets the SHAKES after a really tumultuous experience and that will color our perception of the character's inner emotions too. It certainly adds a new dimension to what we've written and expands the VISUAL language of the character on film.
I hope this helps. I wish you luck with your writing.... and would love to read your work when you've finished a draft.