As a parent of a current student at Princeton, I’ve heard students do both. My son just answered the favorites straight-up with zero elaboration (song title with artist, quote with citation, book with author, etc). I know other students wrote a tiny bit on some questions. I would just say to use caution when elaborating. Clearly, you don’t need to add anything if you don’t want to. I would be wary of letting nerves and anxiety make you feel like you have to write more. At the same time, if you have something to add, keep it short.