Parenthetical direction should be treated as a separate element, placed on a line all to itself.  As with all screenplay elements, parenthetical direction should not be centered on the page.  Instead, it should appear in its own margins, approximately halfway between the margins for dialogue and the margins for character cues.  The standard setting puts the left parenthesis 3.6 inches from the left of the page.  The text should wrap under the text, not under the parenthesis.

Parenthetical direction should be short, no more than a few words, and not a complete sentence.  As such, it should not have the first word capitalized (unless it’s a proper name), nor should it end in a period.  If the direction is long enough to merit a complete sentence, then it should appear as an action element.

Use parenthetical direction to indicate a speech is continuing.  When the same character speaks again in the same scene following some action, it’s customary for the word “continuing” to appear in parentheses on the next line after the cue.

It’s also acceptable to place the word “CONT’D” in parentheses as an extension to the character cue, but things can get messy if there’s already a “V.O.” or “O.S.” extension.

Use a parenthetical beat, not just an ellipsis, to indicate hesitation or an adjustment in a speech.  Personally, this reader prefers the use of “(then)” instead of “(beat),” as it flows better and cues the actor that this is an adjustment.

Song lyrics in dialogue should appear in quotes under the parenthetical direction "(sings)."

If some parenthetical direction interrupts a line of dialogue, then the unfinished line should trail off in an ellipsis.  The line should pick up after the parenthetical with an ellipsis, as well:

Under no circumstances should ALL CAPS be used in parenthetical direction.

Parenthetical direction always comes before a speech, never after.  If a character performs action at the end of their speech, it should be written as description or action, not as parenthetical direction.