Interchangeable characters, or casting stock extras

At Boskone last weekend I was on a panel where we discussed minor characters, and how to make them memorable.

I was reminded of the panel when I was reading a book earlier this week, as the protagonist encounters yet another former acquaintance with whom he no longer associates due to his recent disgrace. And I realized that I couldn't tell this person from the other similar characters that had already been introduced. The character didn't have a unique voice, and the few personal details used to describe him share too much in common with other characters. The next time he shows up, I'm going to have a hard time remembering which one he is.

I can't figure out if the problem is that so many of the minor characters all have the same stock feel, or if there are actually too many minor characters to keep track of. Sometimes less is more--a handful of richly drawn minor characters would serve the story far better than a constantly proliferating cast of cardboard cut-outs. As it is, I reached the stage where I needed a scorecard to keep track of who is who, so I've set it aside and moved on to something else.