There aren't hard & fast 'rules' as to what should be capitalized - It depends on a bit of style preferences. Generally these days people just:
1. Capitalize the name of a character in action description - but only when the person is first introduced.
2. A significant prop the first time its mentioned.
3. A significant sound or action
In the old days almost any sound would get capitalized as well, but you don't see it as often now.
It really does depend on style - compare a few screenplays that hit the screens this year. (With apologies for formatting)
Avatar:
We find Jake near the pool table, BALANCING his chair, front
wheels off the ground, while holding a tequila shot on his
forehead. ONLOOKERS, including some other disabled vets,
CLAP and WHOOP.
Jake grabs the glass, SLAMS down the shot as they cheer.
Kick-Ass:
We pull back to see that the car is inside a CRUSHER. Mindy
smiles and hits a BUTTON. The crusher grinds into action.
Momentarily, a small metal CUBE lands at her feet.
Toy Story 3:
Gladiola pulls an old WALKMAN tape player from a shoebox, puts
on the huge headphones and presses PLAY. "Let's Get Physical"
blares out --
She HUMS off-key and starts dancing like a SPAZ.
Any of those examples could have been capitized slightly differently.
Just pick something reasonable and don't get too stressed about it !
Good luck,
Mac
