(Harry Potter Spoiler Alert.. read no further if you haven't read the book)
Professor Quirrell is introduced to Harry Potter first upon his introduction to the Wizarding World by Hagrid.. it's this status/location that allows us to dismiss him so quickly. He's the first, in a long line of new people. He's odd, yes.. but not nearly as odd as others we'll meet. He's meek, he's not outstanding, he's unremarkable in all ways except for one or two areas where he's eccentric.. but compared to the others he's relatively boring. Yes, he stutters, and he wears a weird hat.. but those aren't threatening characteristics. Not like someone else, someone who demands our focus because of their suspicious actions.
Quirrell's are usually passive characters, meaning that the meeting of the Main Character and the Quirrell is a result of an action of the Main Character, either directly or by an introduction. Quirrell's don't usually make strong, affirmative actions because that draws attention to them, and they are much more happy to sit in the background.. unless there is a reason for them to step into the limelight.
(Da Vinci Code Spoiler Alert.. read no further if you haven't read the book)
In Dan Brown's smash hit, The Da Vinci Code, his Quirrell is Leigh Teabing. Brown's Main Character, (MC), Robert Langdon, seeks out Teabing because of his expertise, not knowing that he is the antagonist behind all of the bad things that have happened.. and Teabing is a great Quirrell because he's unassuming and because he is brought into the story willingly by an unknowing main character.
It's his status as being innocently introduced into the story that makes him such a clever antagonist. Whereas Quirrell is introduced first to allow lots of time to pass between his story introduction and the bad things that begin to happen, Teabing is introduced late, so that there is an equal amount of time between the bad things, (that start right off in the book), and his introduction.
That misdirection on the timing of their introduction is a great way to hide their importance. Whereas the red herring character in both stories is introduced much closer to the bad acts. Snape is introduced when Harry first feels pain from his scar, Silas, (the albino killer in Da Vinci Code), is introduced early on as the puppet that carries out the actions of 'the teacher', (Teabing.)
Another way to throw off the scent of the reader.. give your Quirrell a frailty. Quirrell has a nervous stutter.. Teabing is old and walks with a limp.. the more you can do to make them seem harmless, the better the shock will be when they're finally revealed. Think of M. Night Shyamalan's movie, 'Unbreakable'.. (Spoiler Alert).. the bad guy is revealed as the man with the disease that makes his bones brittle. He is physically weak, but that weakness hides his status as the antagonist to the protagonists 'Super Hero-esque' strong body.
In that way, Mr. Glass, (M Night's antagonist) is connected to Bruce Willis's hero because they're both one end of the physical spectrum.. one physically weak and breakable, the other physically strong and unbreakable. That's another way to disguise your Quirrell.. make them like your hero and/or make them 'above suspicion'.
Quirrell is meek, seemingly scared.. much like Harry at the start of the story, plus he's a Professor. Teabing is a history/grail fanatic and a wealthy, well connected politico. Mr. Glass is the knowledgeable trainer of Willis's super hero and the only person who believes that a super hero must exist.
So.. to boil all that down for you.. the ways to create, (or spot) a Quirrell are:
- Introduce them away from the 'bad acts'.
- Make their introduction passive on the Quirrell's part, (unless your Quirrell is seeking the hero ala Mr. Glass)
- Make your Quirrell appear unremarkable, average, common.
- Give your Quirrell a frailty or perceived frailty.
- Make your Quirrell like/connected to your MC, give them a commonality.
- Make your Quirrell a member of a class that is above suspicion. (ie.. Professor, Wealthy Politico, Knowledgable trainer)
Now, go create your own Quirrell.