I wonder sometimes if this happens when the writer's subconscious lets something loose, what had been restrained by the more natural, habitual main character.

Yeah; I think for many authors the parameters of a protagonist are more restrained, so all the really weird stuff precipitates out into the secondary characters, who don't come with the same expectations.

So, who came to mind for you?

Well, I was provoked by Prewytt Brumblydge and Dean Priest, but my liking for the supporting cast goes back to characters like Fflewddur Fflam in Lloyd Alexander's Prydain Chronicles, Sempitern Walker in Dianna Wynne Jones' A Tale of Time City, or Puddleglum in C.S. Lewis' The Silver Chair, all of whom I encountered in childhood and who made an impression on me one way or another—I leave out characters like Schmendrick (The Last Unicorn) or Howl (Howl's Moving Castle) or Sydney Carton (A Tale of Two Cities) who fall into the much rarer classification of weird protagonist. But I've got a list that goes on for pages. I'm the person whose favorite character in Aliens is Bishop, or whose favorite scenes in Dr. Zhivago are the ones with the doctor's cynical half-brother Yevgraf. Pooh-Bah gets all the best lines in The Mikado.