Scripts are generally written in non-formatted courier text. So you don't really italicize or bold anything. This is changing somewhat in terms of the use of italicizing and bolding, but for the most part, you should just not bother. You should use ALL CAPS for things that require attention from other collaborators downstream (e.g. RINGING BELL) would be capitalized to make things easier for the sound designer. You should also ALL CAPS specific visual impact items to underscore their importance. (e.g. the helicopter SMASHES INTO THE SIDE OF THE BUILDING).
All that said, there aren't exact rules on using styles in scripts. Clarity is always the most important thing.