To preface, yes DID is usually protrayed in a bad way by the media, yet Shallan's instance of DID is quite different lest I don't even know if you could even label her as having DID. I'm no phsycologist, only a teen speculating on the internet, though I have read articles and watched videos about the disorder. DID is usally caused by trauma, which Shallan has yes, though most people with the disorder black out before changing into said alternate personality like Treamayne said.
I've seen an interview where a person could even make the conscious choice to bring out one of their alters, yet they still blacked out, and their personality and behavior changed completely. The person demostrated the transition for the interview, though for the most part, they coudn't control when a stimulus triggered them on a regular basis leading them subconsciously change into an altered state. Heck a person with the disorder could be doing the dishes, but since water is one of their triggers they could change personalities without the willingness to do so.
Additionally, a person with DID may feel quite disregulated, and they might have to find ways to communicate with their altered personallities (via writing things down and what not) in order to get things done. Shallan doesn't black out or wake up in a completely different place with a distorted memory of what happened, rather she brings out another side of herself to help cope or handle issues.
P.S. - If I got anything wrong let me know.
P.S. x2 - I should have mentioned that people don't purposefully create their alters like Shallan did.