Writing books can sharpen your powers of observation in other media. On a recent television
program the story involved an unknown person sabotaging and undermining a project. But who? It could not be one of the 'regulars' as that would have meant the demise of the character on the show as the acts were too extreme for that to happen and the show's premise continue. Sometimes the perpetrator just has to be the character who otherwise seems unnecessary. Almost too obvious.
I don't think that happens as often in books as there is no cost other than the author's time, involved in introducing and 'maintaining' multiple characters. There can be difficulty for the reader if too many characters are brought into the story but it may be that the author wants to muddy the trail and introduce red herrings to cite a couple of idioms. The reader must spot the right tree in the forest. A glossary of characters isn't used very much anymore; I recall the last one I saw which was in a book with over thirty characters. Let's just say I referred to it often.
When the perpetrator of a crime is revealed in the last chapter, the reader immediately congratulates himself for having figured that out or feels annoyed at having erred. Then the next action is to page back through the book, and memory, to see if the clues were there had one only been sufficiently attentive. If the hints were sparse or even missing, there can be a definite sense of being misled and tricked as if the game wasn't fairly played.