Generally, text in a screenplay’s actions or stage directions will be in ALL CAPS when something deserves special attention.
Some common all caps examples:
The first time a character is introduced, his or her name should be in all caps. This is the one steadfast all caps rule.
Visual or special effects will often be in all caps.
Specific, important sounds that either have to be captured during filming or added in during post.
Any integral or specifically important props, wardrobe, or other details the first time it is mentioned.
Anything else the writer wants the reader or audience to pay particular attention to, or the writer wants to be certain the director, effects artist, editor, or any other crew member notes.
Other non-stage direction instances worth mentioning:
All stage direction in multicamera teleplays.
Transitions like FADE IN:, CUT TO:, etc.
Act/scene numbers in teleplays or other formats that demand these.
The speaker’s name above his or her dialogue.
Abbreviations such as V.O., O.S., etc.