We run across fiction writers who want to write multi, now and again. Some of them openly ask for advice from multiples, others seem to want to but are afraid of appearing rude or presumptuous. Regardless, this page is for them.
Note that this is not a page on how to write Dissociative Identity Disorder or Multiple Personality Disorder. If you want to write a strict iteration of the DID/MPD "healing journey" narrative, you've got plenty of examples to follow; seriously, just look in your library's catalog for 'multiple personality' and choose one. This page is about writing multi more broadly, as the experience of more than one person in the body. It will guide you through some of the cliches and pitfalls associated with writing multi, and how a system's experience in narrative may differ from a singlet's.
Chances are you'll disagree with something on this page. That's cool. These are meant to be guidelines, not an ironclad Law of the Multi. We all have our own personal biases and pet peeves. However, we've read a lot of books on multi, and we have some writing chops, so hopefully we have something useful to say.
The most vital thing I can say about writing multi is don't think of it as 'a character with multiple personalities.' You've already lost the battle. These are multiple characters sharing a body; you need to flesh them out and give them goals and conflicts and back-stories just like any other character. To use a paper-thin stereotype as a system member under the guise of 'this is how personalities are!' is lazy storytelling. If you're going to write a system member with no more character than 'adorable lisping child who likes ponies,' you better be prepared to say why. Is Ponygirl withdrawing to a shallow template to avoid a traumatic experience? Is she being purposely stifled by another system member who fears her latent power? Is she faking it so as to pull the wool over the eyes of the adults so they don't realize she's planning to overthrow them?
You might get thrown by this; shallow system members are a dime a dozen in the literature, so surely that's the right way to do it, yeah? No. I'm sure you can think of a common stereotype in any genre that you hate--I myself would be happy to never see another Amoral Broody Sexy Vampire ever again. Do you really think most multiples enjoy seeing themselves flattened into 2D? We don't.
Another thing to keep in mind is that a system has to function as a gestalt. We can't just run around willy-nilly; without some form of organization, nothing will get done, and this goes double for folks with low co-consciousness. A system might run like a family, a co-op, a corporation, or a tyrannical dictatorship, but there has to be some kind of guiding rules--though note that depending on the system, some members may follow them better than others. I suppose you might want to write a system being extremely low-functioning and flaky, but... it's been done. A lot. If you can find new territory to explore, great, but we would frankly find it refreshing to see systems who actually function.
Note that the system rules don't have to be nice. A corrupt system government could have a lot of mileage as members gang up to overthrow a tyrant. Or someone might attempt a hostile takeover. Or perhaps a system of favoritism and bribes, with some people left in the cold. You could write intrigue and court politics, if it strikes you--or enjoy the challenge of a system with low co-consciousness who keep track of everything with a Smartphone and Google Calendar. The possibilities are endless.
Then you have world-building. Some systems have no headspace, which is fine, but if you want to create one, the sky's the limit. From a simple, surreal dreamscape like ours to entire planets and galaxies, all are perfectly feasible, and you can rewrite the laws of physics to suit your system. Many system members have lives out front and inside. Just because Ponygirl doesn't front much doesn't mean she's inactive inside. Just because Ponygirl doesn't front much doesn't mean she's inactive in headspace. She might play in a stable, or do chores around the headspace. These are all things to think about, if your system has a headspace.
If this sounds complicated, that's because it can be. Depending on the system, you might find yourself working double-time, building worlds, plots, and characters both internal and external. Or you could keep it a simple system of two with no headspace and a very informal rules agreement. You have no shortage of options, and most of them are unexplored.