It’s important for readers to be able to distinguish the special characteristics of all the meaningful players in your story, not just to recognize them when they show up, but as yet one more channel to provide potential insight into their character, thoughts and their likely actions.
Of course, you can construct your “man of mystery”[tm] whether as a main or bit part as an opaque personality, but really, isn’t it more fun to allow your mad quirkery out to play instead?
I look for what involuntarily engages such a character’s attention, something his brain says “Ooh! Shiny!” about, even if he sees it inconveniently at a moment of peril, when the notice thrusts itself distractingly upon him.
Think of the evil genius, stopped in mid-gloat when she notices her captive is wearing the latest fashion, and she wants to know where she got it.
Or the sinister dog-fancier, distracted by an unusual coat color on a stray, so that he spins around from his path to check it out, abandoning his frantic hostile pursuit of the hero for something more important.
Maybe they sing, Barbershop enthusiasts, and gravitate to listen to a quartet performing when they should have been running away instead.
Maybe the villain is made suspicious about the bona fides of the hero because he detects an improbable claim — “he can’t have actually found the Holy Grail”. (“But if he did, I want a look!”)
A really savvy protagonist can construct a suitable specimen of obsession as a planned subversion of a susceptible suspect, as a lure or a distraction.
I bet you have some examples of this sort of thing, either ones you’ve created or ones you’ve admired from others. Come share in the comments!
Mad Genius Club 
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