Every writer learns that it’s important to create three-dimensional characters. The people who come to life in books must occupy fictional space in unique and interesting ways. But because most stories are about a protagonist’s personal journey–even if that person never leaves the backyard–characters should truly be four-dimensional. The character at the end of the novel, short story or picture book ought to be different from the one readers met at the very beginning as circumstances have their effect.
Naturally, there are exceptions to this. The two boys in the CAPTAIN UNDERPANTS books didn’t change one bit in the first six books. And I still don’t know who is who unless I check the opening page and see that “George is the kid on the left with the tie and the flat-top. Harold is the one on the right with the T-shirt and the bad haircut.”