The guidelines: keep head hopping to a minimum; collapse characters that could do the same job into one; as few viewpoint/main characters as possible; don’t introduce viewpoint/main characters very late in the story, such as in the third act. But why are these important guidelines?
To write a novel is to guide your readers through an emotional experience. You want to do so as smoothly as possible, using every tool in your possession to keep them on the track you want them to be on, emotionally. One of the primary tools for doing so is characters.
Each character that is important has their own journey to go on, and the reader is following them, their guide on that emotional experience ride. If there are too many guides, the reader will be going in too many directions; if they start showing up just when the ride is getting really exciting, pulling the reader in different directions, things will be very bumpy and they may even be thrown entirely out of the park.
There are many guidelines that editors and writing advice books will offer you, and frequently you’ll say “But my favorite novels throw that guideline right out the window!” You’re probably right. That’s why I try not to refer to them as rules. But here’s the thing: your favorite novels? The author has almost certainly broken only a few of those guidelines, and for a really good reason and – this is important – with great skill and ability. New authors tend to flail about, ignore the rules, smash together whatever they’ve seen work well in other novels, without a strong understanding of why that broken guideline worked so well in that novel. And without understanding that breaking them all is simply going to lose readers’ attention.
It’s not that the guidelines should be unbreakable, even for new writers. But to learn when to break them mindfully, knowing what you’re doing, you first need to follow the guidelines, understand them, internalize them, accept why they work. Only then can you tamper with them.