in screenplays there are usually little details that are deeply representative of a character's personality so I want to know what little actions and objects would represent someone who is crazy and quirky on the inside but acts quiet and somewhat depressed
Everybody nowadays seems to think they're "crazy and quirky" because they can't perceive others' similar magnitude of idiosyncrasies. These qualities are but measures of deviation from the norm. Since no precise definition is given of what's unusual anent your character, I've no idea what sort of physical manifestations you could ascribe to them. In general, it could be anywhere from the clothing, hair, manner and subjects of speech & writing, expression of emotions, possessions, habits & hobbies; all the while keeping up with expectations of formality (to some extent) - studies, work, social clique, recreation.
If we're talking live play, I've seen this neat stage technique where onscreen actors would freeze, save for the one character who just monologues their inner world to the audience without the others noticing (or possibly breaking the 4th wall with a minor reference to it now and then). Mayhap dim the lights to emphasise the effect to the viewers.
If you are using narrative (all knowing story teller) style, you can simply explain that. For any other style it might take some cleverness.
If you want to write, you need to read novels. Lots of novels. Any novels. Notice that they all have perfect spelling, perfect punctuation, perfect composition. Notice where they use one space and where they use two. Notice that every speaker in a dialog gets a new paragraph, and every paragraph is preceded by a blank line. Notice any writing techniques or figures of speech. Take notes so you know where you found what you know. It takes a lot of reading to be a writer.