Opening Credits Cast Party

Hey c'mon, we're about to put on a show!

You might never see them all in one place during the show itself. They move in their own circles and follow separate storylines. Yet there they are every week, all these people that are somehow connected to each other, standing there self-consciously with fixed grins and arms around each others' shoulders for a big gang portrait, like they're at a family reunion and someone broke out the video cameras. Are we looking at the actors, or the characters? It's kind of hard to tell.

If they're not just standing there, they're on the move, walking together like a chorus line, and that blurs things even more. They're all dressed and made up like their characters on the show, but why would they be all walking together, and so purposefully? It feels like the camera has caught the actors heading for the set at the top of the shooting day. If it's particularly heroic, it's a Power Walk, but it might just be the cast walking together in a tight group in exactly the sort of fashion you'd never see them do on the show itself.

What's perhaps strangest about this trope is that, whereas the Title Sequence is most often compiled by raiding your own show for clips, these awkward group portrait or kick-line moments usually have to be filmed specially, on purpose. The other big problem here is that if the cast changes, you have to reshoot the cast party with the new cast, sometimes over and over again—perhaps unpleasantly drawing attention to missing faces and unloved interlopers.

To fully qualify for this trope, the credits cast party needs to show the cast artificially grouped together for the camera, essentially breaking the Fourth Wall, rather than just interacting as they would on the show. Bonus points for getting all the cast together even if some of them would never be together in the show.

Supertrope of the V-Formation Team Shot, where the Gang of Five poses in a heroic V. Compare the Team Shot, where the gang are posed together at the end of an anime.

Examples of Opening Credits Cast Party include:

The original Beverly Hills, 90210 began doing this at the top of its opening credits starting with the second season, ganging its cast in front of a photo-shoot-style white background, laughing with each other and smiling for the camera. (Of course, this being Beverly Hills, it's not too hard to believe that bunches of beautiful teenagers are doing photo shoots together every day of the week.) They carried the trope forward for the rest of its ten-season run, and given the constant cast changes it had to be reshot on a regular basis, seeming less and less spontaneous each time, until it was finally rammed into the ground with the many-times mutated cast milling around looking like they were just hanging out waiting for the series to finally die.
Similarly, the opening credits of Popular featured the entire main cast at a photoshoot together, despite the premise of the show being that the two main groups of characters hated each other, and the unpopular, brunette clique weren't exactly the photoshoot types. Bonus points for including the two 'parent' characters who, as the show went on, were rarely seen outside of the opening sequence.
Law and Order and some of its spin-offs routinely end their opening credits with the four main principals striding purposefully down a corridor together, presumably because that's where the courthouse vending machine is.
It's a Living/Making a Living started off its credits with its waitresses all joining up to merrily walk arm in arm toward the big steel skyscraper atop which sat the luxury restaurant they worked in. The show's troubled history and revolving door cast meant this artificial scene was reshot more than once.
The opening of Charlie's Angels originally included the three angels, Jill, Sabrina, and Kelly, walking together toward the camera partway through the credits—the premise being that the three left police work together to work for Charles Townsend. When Jill was replaced by Kris (Cheryl Ladd), this was reshot with Sabrina and Kelly walking together, and Kris coming in from the side to join them. And so on.
The sprawling cast of Soap required three cast pictures in the opening credits—the Tates, the Campbells, and all of them together. This had to be redone more than once because of cast changes, resulting in different gags: first a fight breaks out among the characters, then another version had the ceiling collapse on them as they stiffly pose, then a return to the fight idea.
The second round of credits for Eight Is Enough involved, somewhat plausibly, the Bradford brood forming a human pyramid under the show's cascading title, which then collapses. But the real awkward posing comes just a bit later, as the odd number of alphabetical-order "other" cast members ends up putting Willie Aames backed up against a wall pretending to share a laugh with two of his on-screen siblings, Dianne Kay and Connie Newton.
The beginning and end of the Growing Pains opening credits showed the Seavers standing together outside their house for a family portrait. The reprise changed every year, as the characters went back into the house leaving one of them to mug for the camera.
Full House gives everyone their own introduction shot. But by the end of the credits, they're all goofing around and having a picnic together.
After the first season, Family Ties avoided having everyone posing awkwardly together by having an anonymous hand working on a painting of the Keatons posing awkwardly together.
The opening of Friends has the characters goofing around a fountain in the park. The odd part isn't that the characters are all together (they almost always are in the show itself), it's that they're apparently having a late-night party in the middle of a deserted park... that has a couch!
- And a lamp that's hooked up to the fountain's floodlight!
  - Fans have attempted to shoehorn this into the show's canon by saying that it was a photo shoot that Joey was participating in early in his acting career. For some reason, they needed five other extras in a hurry, so Joey called up his friends.
Certain later seasons of Mystery Science Theater 3000 showed a shot of Mike and the Bots waving to us from the viewport of the Satellite of Love. But then, MST has No Fourth Wall, so it's kind of a trope subversion (as well as a parody of a similar shot at the end of the credits for Babylon 5).
The opening of Lizzie McGuire has the main cast playing with a ball in front of an abstract background.
The early seasons of Roseanne followed the family around the dinner table. In later seasons the opening had to be changed due to the replacement of a main cast member. They forfeited the Opening Credits Cast Party in favor of a montage of time-progressed photos of each cast member as they were credited.
- Which lampshaded the cast change.
The Cosby Show: The first season was a photo montage, but subsequent seasons had increasingly complicated dance numbers featuring the cast dancing to various musical takes on the theme song.
Family Guy mocked this at one point with their fake ad for 'Shoving Buddies', the new show coming this fall on the FOX network. It had the cast doing this, just this, for about 5 minutes.
Martial Law climaxed its season one opening credits with the cast walking purposefully towards the camera (which had to be redone twice - the first time because Tammy Lauren left, and the second time due to Arsenio Hall joining).
A rare animated example (as it's too sedate to be called an Action-Hogging Opening): Garfield and Friends. Because the two featured segments (Garfield and U.S. Acres) were very strongly segregated, this was usually the only place the characters from both of them could be seen together.
Most openings of Detective Conan / Case Closed feature really big Cast Parties - the detective kids, the Mouris, the Tokyo Police, Hattori and Kazuha, FBI, end so on.
Three's Company showed the entire cast spending the afternoon together. Even though there is no explainable reason why the three main characters, plus Larry, plus their land lord and his wife, plus the creepy lady who Jack hates would all be hanging out with each other.
Skins starting in series two has the cast playing around and having fun. Of course, so they won't be like those American teen shows they add the sex, drugs, drinking, and drama so it would be more in character for them.