Basic Trope: A popular character who frequently appears in books, comics, and films with little or no connection with the character, often gratuitous and serves only to sell the book, comic, or whatever.
Straight: Any random villain is fighting any random hero. At a loss for how to beat him, the hero calls on the character, and the two of them beat the villain together (as advertised by the cover).
The popular character has a starring role in a series that has nothing to do with him, in a story that has nothing to do with the series he is in.
A whole crowd of popular characters from different continuities all appear on a comics cover but none of them are in it.
A well-known superhero makes a cameo in a series where superheroes play no part in the plot and may not even exist.
Downplayed: He does appear in the latest issue of a comic book, but only as a background character in a big crowd scene, and it's not drawn attention to in the ads.
Justified: Because the character is so powerful, he is considered the most powerful character in his respective universe and so other characters frequently go to him for help.
All of the character's team are called to help the main character of another book, except for the popular character.
Alternatively, Obscure Man shows up in the popular character's book.
Or: the character is conspicuously absent from an issue of his own book.
Subverted: The heroes go to great lengths to contact the character, only to find he's not home.
Double Subverted: ...Because he was already kicking the badguy's ass!
When the character can't show up physically in the comic, there is a large poster of him in the background.
Alternatively, the poster gets a splash page. This poster may be discussed at length, also.
Even still, he gets a little cartoon at the end saying "Obligatory Cameo!"
Yet another; the character gets a random scene featuring him getting a coffee that has nothing to do with the rest of the story. The hero might even say "What the hell was that about?"
Averted: No popular character other than the regulars show up for an entire book's run.
Enforced: The most common: A comic or series is losing viewership, and as a last ditch attempt to boost sales, a popular, well-known character is brought in as a guest star.
Lampshaded: "This guy's pretty tough. Let's call Captain Bob! He's used to being called up on a whim, anyway."
The heroes have the character on speed dial, because, due to his nasty personality, he needs to team up with lots of heroes so that the public won't mistake him for a villain.
Alternatively, they have him on speed-dial because he's so anti-social that they need to constantly stick him in team-ups for his own personal growth.
Defied: "Hey, guys, let's call Captain Bob, hyuk hyuk hyuk!" "No, you idiot! We never call Captain Bob to do important missions!"
Discussed: "Ever notice how Captain Bob appears in, like, every comic these days? Even the stories he's not in, if you look at the credits he made cupcakes for the team or something!"
Conversed: "Captain Bob? What are you doing here?" "I'm contractually obligated to show up in everybody's comics. Yup, time's up."
Deconstructed: In a multi-story crossover event, the popular character has to warn every single character that a super-villain is going to attack, and appears at length in all their comics to tell them, and sometimes, to ward off the villain's Mooks.
Played For Laughs: The popular character, after years of making cameos in everyone else's series, decides to quit making cameos. However, as he tries to avoid all the usual heroes, the battles somehow get taken to him. Hilarity Ensues.
Played For Drama:
The character feels he is being taken for granted by the superhero community, and finally throws it in the faces of all the other characters that, without him, they could never have gotten anywhere. This causes the character to become an outcast from the community, and, sometimes, hunted by them.
Powers or no powers, the popular hero has his limits, and he's wearing himself out trying to be everywhere at once. By the time he gets home, he's battered and exhausted... And his Arch-Enemy is there waiting for him.