I don’t know that I always setup my characters very much in the text, but I always have a very clear idea of who the characters are.
For example, I just wrote ‘Never Had I Ever’, and though I never mention that Nick is a heavily closeted gay dude and Will is a bicurious guy who likes to play around with him, it’s implied in the dialog and the bits of history that characters reference. I don’t know if any of the readers picked up on those two character’s sexuality, but I don’t really care, it’s just important to me that there’s a reason why they act the way they do and can reference the experiences they’ve had. It makes their actions flow from who they are, rather than because the plot demanded them.
Same story, I established early on that Peter was a year younger than everyone else and had been having a hard time with the isolation. That meant when he suggested playing the game, the other characters got on board with it not because the plot needed it to happen, but because they felt sorry for their buddy and wanted to cheer him up by doing something he suggested. Less than 10% of the way into the story, they’re playing the game and involved in the mechanic of the transformation, but they’re doing it in a way that seems plausible.
I almost never write with a plot fixed in my mind. I like to setup some characters, a situation, and a mechanic for transformation, and then I just follow it where it takes me. Of course, my brain takes it to areas that resonate with me, whether sexual or emotional.
I absolutely hate the paragraph’s that are like “Joe was a twenty-three year old white male, fresh out of business school and trying to do well at his stressful job as a lawyer. He was 5’10”, with a thin build that told you he rarely worked out. His hair was brown, his eyes were green, and his 7" dick was uncut. He wore a three piece blue suit which hung too large on his frame." It just feels hamfisted.
I have also been known to use paragraphs just like that, usually in the context where the character is almost breaking the fourth wall with an awareness of the fact that they’re in a story. I recall doing something like that in “Hey, you’re straight, right?” when the narrator describes himself and his friends before the transformation. I’m far from a saint when it comes to following my own rules.
I like when physical descriptions are mentioned in context. A character is watching someone from across a room, and admires their body. A character looks themselves over in a mirror. A character gets made fun of for a distinguishing feature. You know, the times in real life when he actually notice our physical attributes. I wouldn’t want to read a list of the characters measurements, but I could buy it if the character is a narcissist to loves checking themselves out in the gym mirrors.
I don’t like to include story that doesn’t directly support the sexy bits. Like I probably wouldn’t spend 500 words discussing how a character had a hard time with being bullied at school, unless the story is about him getting back at his bullies or becoming one himself. That sort of thing.