Honestly?

Hey, I bet (character) would (do thing). Seems like they would.

I bet (ridiculous concept tangent)

Lemme establish a place for (thing)

Hmm... who else would do (thing)

(Proceeds to create background characters with their own lore to accompany primary character)

Example, Cheeks:

She was created because Chrys and Vance are gym rats. Vance is usually strength training or sparring with Mike, so Chrys would be on his own because he's more lithe than strong. Chrys doesn't handle loneliness well, so Cheeks was created as his overly peppy, increasingly obnoxious fitness instructor. The rest of her story developed from interactions with Chrys, Mike, Vance, and a few other characters.