I have been reading two shounen manga lately, Bleach and Naruto, and have been struck by the ways that both handle a sudden expansion of the cast from a relatively small (but still ensemble) group to a very large one. This is of great interest to me, as several things I'm working on have large ensemble casts, so I wanted to analyze what makes this sort of story succeed or fail.
Note: I am going to write about this without spoilers for those series, other than to mention that there are arcs called "the chuunin exams" and "the Soul Society" that introduce a lot of new characters, and to speak in very general terms about how that works. Please do not leave any spoilers beyond that in the comments. Ie, it is fine to write, "Rock Lee's introduction was a great example of how to set up a character in one way and then let the reader's perceptions of him shift." Do not write, "It was really well done when [spoiler] won his bout with [spoiler]."
However, if you don't want to hear anything at all, even on a very vague level, about those series past the point where you may have read, this is why I am cutting.
"The chuunin exams" arc is a ninja exam consisting of a tournament of individual one-on-one bouts, with spectators present.
"The Soul Society" arc involves a mission whose purpose I won't reveal.
There are a number of potential pitfalls involved in either suddenly expanding the cast or starting out with a very large one. If there are already characters we've known for a while, the readers will already be attached to them and may resent time taken away from them. The character development of the existing characters may be shorted to give more time to new characters. Readers may not care about the new characters.
If there a ton of characters from the get-go, or a lot introduced at the same time, readers may be unable to forge an emotional bond with any of them. Readers may not even be able to figure out who they are or keep track of their relationships with each other. The entire enterprise may be overcome with a vast sense of not caring.
I think that Naruto handles this problem better than Bleach does. Here's what Naruto does well:
1. Though sometimes a lot of new characters are seen all at once, they are each given long individual introductions at the point when you need to know who they are. You're not thrown into a bunch of tangled relationships-- those relationships are carefully sorted out for you when you need to know what they are. For instance, Neji and Hinata get a big scene together where their relationship is explained. Before it's explained in that scene, they are barely seen and you don't need to know who they are.
The Soul Society arc starts out by introducing just a few characters at once, but it soon starts throwing tons of them at you. Also, characters are introduced who clearly have relationships with each other, but it's not clear till much later what those relationships are. With this many characters involved, this gets incredibly confusing.
2. The existing characters stick around more in Naruto. Characters who aren't fighting watch from the stands, comment, and get emotionally involved-- and the action keeps cutting back to them, so we don't feel like we're losing track of the characters we already care about, and they give us both information and emotional involvement with the new ones. Characters who are literally offstage-- for instance, someone who is injured-- are still present in the minds of other characters, and they keep mentioning and thinking about them. Sometimes the action cuts back to people who are physically elsewhere and not immediately involved with the larger story, just so we know they haven't been forgotten. This makes me feel more relaxed and willing to pay attention to the new characters, secure that the old ones aren't MIA.
Bleach has a big, big problem with existing characters vanishing for long periods, during which nobody thinks of, speaks of, or sees them. This makes me resent time spent on new characters, because it feels taken away from the old ones. Also, when the old characters have no relationship with the new ones, there is no chance to make a piggyback emotional connection.
3. In terms of narrative drive rather than emotional connection, it's best if we know why the new characters are there, and why they're doing what they're doing, and what this has to do with the overall story. In Naruto, the reason they're all there is they're in the tournament. It's simple-- but with that many people, simple is good. They're fighting because they want to win the tournament. Again, simple. In Bleach, the story is more complex, and there were a number of times when people started fighting and I had no idea why.
4. Another way in which Naruto is more successful because it's more simple is that, while the characters grow and change and reveal more facets, the first time they're introduced, it's with a distinctive design, a distinctive fighting style, and a very basic character. For instance, Gaara has red hair and black-lined eyes and the kanji "love" tattooed on his forehead. He carries a huge gourd on his back, has an attack involving sand manipulation, and is a psycho killer. There is more to him than that and I suspect a lot more, but that's enough for an intro.
In Bleach, characters have distinctive designs and attacks, but they all wear the same uniform. They sometimes start out with a single distinctive trait, like Byakuya being "I am too noble and lofty for this world, and by the way I can kill you with my pinky," but sometimes they are introduced with either nothing distinctive or too much complexity already in place-- which makes it harder to get a grip on them.
Short, absolutely unspoilery version of what works:
1. Introduce characters and their relationships with each other one by one or at most two by two. Don't have them do anything important until you've given them a nice long introduction.
2. Don't lose sight of the old characters. Literally cut back to them if they're offstage, or else have onstage characters think and talk about them. Have old characters introduce us to the new ones, and be involved with the stories of the new ones.
3. Keep the story and motivations simple until we are already thoroughly familiar with who everyone is.
4. Give the new characters really simple and memorable personalities and motivations upon the first introduction. Add complexity later, when we're not still trying to figure out who's who.
Further thoughts?