If a character appears in a scene — in the background or otherwise — before he is introduced, it is important to make mention of it. If not, he could easily be left out of the scene when it is filmed. You also want to be sure your reader takes note of the character’s presence.
You do not, however, want to draw attention away from the scene by focusing on one of its more minor characters, nor do you want to introduce that character to the reader as an aside when he deserves a full-blown introduction later.
A good way to balance this is to make passing mention of the character with a “who we will meet later” type-phrase attached. For example:
Tim is flanked by his two pudgy cronies, who we will soon know as MITCHELL and MARCUS.
John August’s advice can be found here.