Pass the buck, young authors.
While you are ultimately responsible for any and all character development, don’t lay the entire burden upon the narrator’s shoulders. A good deal of the heavy lifting of backstory can be handed off to minor and/or inconsequential characters.
Interactions with these often nameless folks can tell your readers more about the protagonist than the longest, sweetest internal diatribe sung by your narrator ever could.
So, throw in a “meaningless” conversation with a mailman. Toss in a noisy neighbor that needs to be calmed. Fling a cat needing to be rescued into a tree and have your man play hero.
Bottom line: Delegate.