Zig Zagged: After three seasons of characters talking about Charlie, it is revealed that he is deceased. However, he is eventually seen as a ghost.
Averted: All characters ever mentioned appear at least once.
It's a biographical work about Alice and/or Bob. Charlie is Bob's best friend, so he has to at least be mentioned, but he could cause network troubles if directly confronted (he's a prostitute, gay, drug addict, etc.) so he is not shown.
Lampshaded: "You know, Charlie never seems to have time for us. I wonder what he's up to?"
Invoked: Charlie tries to visit Alice and Bob, but "Dylan the Villain" (actually an Anti-Hero) chases him off. All offscreen.
Exploited: Dylan wants Alice's friend Elaine to do an unspecified dirty deed. Knowing that Elaine has never met Charlie, Dylan bribes her with a photo of him. (The audience never gets to see the photo.)
Defied: Someone finally manages to force Charlie onto the screen.
Discussed: "I've never seen Charlie. What's he like?"
Conversed: "There they go again, talking about Charlie. If he's so great, why don't we ever get to see him?"
Implied: The characters always refer to Charlie using pronouns (as in "I think he was the best friend I ever had,") in situations where no character who is shown would fit.
Plotted A Good Waste: The original plan was to have Charlie appear eventually, but the executives decide to make him an off-screen character.
Deconstructed/Reconstructed: Not applicable. (This trope is completely realistic when played straight, as the characters would not necessarily notice the fact that the audience never gets to see a certain character.)
Played For Laughs: Any attempt to show Charlie onscreen by the characters will end up ill-fated: He never accepts invitations to anyone's house (maybe he's been called into work), any photos of him will be very blurry, and if he is supposed to appear on television, the set will break down (or the power will go out) just before he appears.