This is interesting to talk about because I don’t really consider it to be a theme of the Stormlight Archive on the level that some people present it. But certainly it is a theme, so it’s not like the question doesn’t deserve to be asked.
It’s not that the series is about people with mental illness—rather, I view it as a series where I’m trying to present the psychology of people in a more realistic way, and make their psychology interact with the magic and the setting. In any given small group of people from our world, you’re likely to find a number of people dealing with issues like this—it’s not that most of the world is “normal” and a few people “broken.” It’s more that we all have a wide range of mental conditions and psychologies, and it’s an area that I find a lot of adventure fiction ignores. I didn’t sit down to say, “All right, what mental disorders can I give my characters?” Instead I said, “I’m going to create a realistic and diverse cast—and so some of them are going to have struggles like these.”
I think in particular something like depression becomes more glaring when you’re expected to save the world. And identity issues might become more prevalent if you could literally change your face to become someone else when you wanted to.
To get this right, I use a three-pronged method. First, research. (My Abnormal Psychology class in college—which I wasn’t supposed to take, but managed to find my way into anyway—was particularly helpful.) Second, listening. Any time I include something in a book which uses a real-world inspiration, I try to be very respectful of people that originate the idea. This is particularly true of something like mental illness. I lean on early readers who can give me advice and suggest revisions. And finally, being willing to be wrong—to be told I’m wrong—and do better.