My favorite minor character -- not even a supporting character, more like an incidental one -- is in The Apostrophe Thief. He's Wadsworth, the sadsack aglet-maker from Passaic, N.J., who deals showbiz memorabilia on the side. That whole book is about small things, bits and scraps that people collect; Marian even calls it her "little" case. And you can't find many manufactured items smaller than an aglet. (Marian had to look the word up in a dictionary.) That whole collecting scene was fascinating, especially the odd pieces of info dropped in here and there (such as the fact that Ronald Reagan's mother signed his publicity photos for him).
Ah, but this topic is about The Fourth Wall. Sorry.