From what I've read the difference depends on length and character. Basically the parenthetical guidelines are this: If the character speaking can do it in under four lines, it's parenthetical. If it involves another character in anyway or is over four lines, it's action. For instance:
-----------------
RACHELLE
What's up?
(pause, examines the
scene and appears to
mentally debate whether
or not to continue)
I should probably go, huh?
-------------------
It's fine, but if I added one more line it would have to be a parenthetical. (obviously I shrunk to allow for actual parenthetical width, I'm probably off a quarter inch or so, but you get the gist of it)
-------------------
RACHELLE
What's up?
Rachelle pauses and appears to mentally debate whether or not to continue speaking. She shrugs and turns around.
RACHELLE
(walking away)
I should probably go, huh?
-------------------
And it doesn't necessarily have to do with the way the dialogue is said, it can include actions that compliment the dialogue. Under "How to Format a Screenplay" in the "Writer's Resources" tab there's A GREAT EXAMPLE SCREENPLAY that helped me a lot with some of the basic stuff.

NaNo08 - Darkside - Won
Screnzy09 - Darkside - Won
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