When you have a big cast of characters for a novel, you have a big set of challenges ahead of you. The first of these is deciding who your main characters are. This sounds like it should be obvious and easy to answer, but I know from first-hand experience that you, the writer, can be very, very wrong about which people your story needs, and which storyline actually works for the characters.
Sometimes you have to write a chunk of the book (or at least a few scenes) before you get a real feel for what/who works and what/who doesn’t. My personal rule of thumb is, if a character just flows out effortlessly, that’s your main character, or at least one of your primaries. If a character you plan on being a primary figure in the storyline is difficult, frustrating, or no fun to write, CUT THAT CHARACTER!
Let me tell you a fun little anecdote about my upcoming NaNoWriMo novel. I came up with the initial concept about thirteen years ago. Yes. Thirteen years ago. I started the book five times, got about ten chapters in, and realized it wasn’t coming together each time. So I’d stop, work on other projects, and do some world-building for this novel on the side. Whenever I’ve finished a short story or a draft of my other novel, I’d come back to this one. I talked to some of my writer friends about it. “Cut your main character,” was their advice. Cut my main character??? But she’s the main character, right???!
This summer, between drafts of my Erica Flynn novel, I sat down and looked over my notes about my thirteen-year project. And holy heck if I hadn’t modified the storyline to the point that my main character had become entirely unnecessary to the plot! I’d been writing her out of the book for years, subconsciously. I didn’t enjoy writing the scenes that focused on her, I didn’t like her much (although I admired some of her personal qualities), and I wasn’t inspired by her. The characters I’d written the best material for were either secondary to her, or pitted against her. These are now my main characters. My original protagonist is gone, not even a bit part.
Go with your instincts. Who do you enjoy writing about? Either you enjoy writing those parts because they’re really good parts, or you’ll write them really well because you like writing them. No matter which direction that cause and effect goes, you’re going to end up with better material.
Also, write up a list of all your characters, and write out each one’s “through line” for the book. What changes about them – whether it’s internal or external? The characters who change internally and externally are your strongest, automatically. Those are your main character nominees now. Tweak their through lines. Make them stronger, more dramatic, more interwoven with the overall plot. Play around with it! Have fun! No, I’m not being sarcastic. Really – have fun with your writing. You can be miserable later, when you’re revising. Hah! 😉