The ensemble cast is what keeps readers/viewers coming back for more, every week (on TV), every few years (in the movies), or every few months (in books). What makes an ensemble cast so charming?
First, let's look at a few ensemble casts I see as being near-perfect.
The cast members must have chemistry. By this I mean they've got to have that spark when they interact. They must make me BELIEVE. (Alan Young of MISTER ED never gets enough credit; he made me believe the HORSE IS TALKING! That's what I mean when I say the actor makes me buy into it and I *believe*.)
My husband hates any show I like (and pretty much does it for spite, IMHO--LOL), so he hates EVERYBODY LOVES RAYMOND with Ray Romano and cohorts. But even he has to admit that this ensemble cast is nigh-unto perfect. Note-perfect with the relationships and with how they interact. I *believe* that Robert is the neurotic cop. I *love* Frank (did you realize that he's the one who played Frankenstein's monster in "Young Frankenstein"? They pay homage to that in one of their Halloween episodes!) And I would normally HATE any woman who acted as Debra is often forced to, because I just hate bee-yotch characters . . . but she HAS to get angry and be mean very often in the show, and I find myself saying, "Yes! Deb is in the right! No one could do any differently!" And thus I like her character, against all odds. I even like the interefering mother-in-law, Marie, because she is letter-perfect at being like my own mother and mother-in-law when they got That Way (not all the time, like Marie! Just occasionally.) The cast has chemistry and I *believe* that Robert and Ray are brothers. Watch a couple of episodes sometime and you'll see what I mean. There are almost no false notes (not even with Amy and her parents), and that's why I rank this one up top even above I LOVE LUCY (another ensemble that would not work without all four of them) and THE BOB NEWHART SHOW (the original) and even THE MARY TYLER MOORE SHOW ("You have spunk. I hate spunk.")
My own ensemble casts, I hope, are like this one. I think Ari and Zoe French are a duprass of sorts; there are fans who want to see them in action again ASAP, and even fan fiction being written about them. They function well and are a sort of Bickersons pair. Compare to Jackie Gleason and Art Carney. When they get into it together, Katy bar the door!
Another of my ongoing ensemble casts encompasses Jacquidon Carroll and her sister Chantal. They are more like Lucy and Ethel, it has been said often, and they don't argue as fiercely as Ari and Zoe, but they complement one another when it comes to sleuthing. They have an ongoing foil in the form of Chantal's long-time main squeeze, who is referred to as "Smedley" most of the time. He's like Mrs. Columbo in that we don't see him, we don't hear him talk on the phone, but we hear about him and what he's doing or what he is about to do. It's part of the comic relief. It's a schtick, sure, but (I hope) a good one.
Often your ensemble cast will include a boyfriend/girlfriend or serious marriage prospect of the moment. I have this situation in both LOVE IS THE BRIDGE and in LITTLE RITUALS. If you don't have this, then you'd better have all sorts of flirting and the like going on with one of your main characters, because it's a big draw. Make the decision in the first book as to whether you will have explicit nookie scenes or will draw the curtain and let the waves crash onto the beach as in FROM HERE TO ETERNITY, when we didn't get to see Burt Lancaster's hoohoo. Oh, well. But readers have distinct preferences as to whether they think a cozy should have hardcore scenes or not, so decide right away so as not to anger readers who skip that sort of stuff because they're older than fourteen and don't need an instruction booklet (can you tell which type of reader I am? Except for Henry Miller. I make an exception for Henry Miller. Don't ask me why. But if you want porn, you couldn't do better than Henry Miller, because he has interspersed sex scenes with his philosophical musings and other activities. Try the SEXUS/NEXUS trilogy. Or try D. H. Lawrence. If you want class with your, um, *ss, they're the ones, I guarantee. *Unless you can find a good translation of Baudelaire.*)
Other members of your ensemble cast will probably fill roles such as Sidekick, Mentor, Jester, Lovable (or not) Rogue, Guide (Spiritual/Moral Compass or Tour Guide), Guardian (or Threshold Guardian), Shapeshifter (this can be metaphorical, and generally is, at least to some extent), and even Trickster. Chantal and Zoe are (purportedly) the sidekicks in my mystery series books. Mentors come and go in all my books, but will be apparent when they first appear (so to speak, or "soda speak" as Kevin Robinson, mystery novelist extraordinaire, would say.) Gil Rousseau in MARFA LIGHTS is a Lovable Rogue--Tour Guide who is also possibly a Threshold Guardian and could even play Trickster in a pinch. Tricksters are a lot of fun to write, but you must be certain they're not taking over the story. Your heroine/sleuth/whatever MUST solve the puzzle for herself, save herself, have an epiphany of her own, experience growth and change at her own pace. Someone else can't do it for her, or the reader feels cheated.
So, anyway. You need to make sure your ensemble cast functions well together so that your readership will want more and will come back for more.
Your setting will often (ideally) be a character and will shape the way your cast interacts and what they do for fun (you ski in Denver, but you go tubing in San Marcos (TX) and you surf in Santa Barbara.) The best settings are the ones with a personality of their own and eccentric residents that are already familiar to your readers. For example, in New Orleans your characters will encounter street jazz musicians (and jazz funeral processions!), French quarter tourists, and voodoo priestesses. In Marfa, Texas, you will get the typical artists' colony weirdies plus the Marfa Lights followers. It's also a Western town, so you might even meet a few cowboys in their Airstream trailers. Consider your setting carefully, and be sure you do your research. A few road trips might be useful in this endeavor.
Next time--more about the archetypes and minor characters, and how they fit into this scheme.