Since December of 2008, when I started writing, this habit has kicked into overdrive and I've noticed more quirks, or people have just gotten more weird. It's a toss up. First, let me start with the odd things I do so that no one thinks I'm picking on them. This is for character development after all.
I say "actually" all the time. I don't like it, I try to stop, but it is a habit born of my smart aleck childhood. If I get frustrated, I stamp my foot. Yes, actually (oops, there it is again) stamp my foot like a little kid. I cry very easily - like at Kleenex commercials.
Now, put those characteristics into someone who is a boss, and you will either see their employees disrespect them if they allow it to show, or amusement when it sneaks out very rarely in a usually controlled personality. The latter is true for me. I control my quirks well. On a very rare occasion, one will slip out and generally, people laugh. It is hard to write a character that has characteristics that they choose not to show very often. You have to set it up early that they do it, then show them in situations where they've controlled themselves, then let it fly at just the right time for effect. Otherwise, it's just a character with an annoying habit that does nothing for the story.
If you put habits into a story, it is helpful to have the character struggle with that habit, because we all do in real life. (If you happen to have a really cool habit, then you work on doing it more often, so still a struggle).
Comedians are a great resource if you are wanting to come up with something that sets your character apart. They've done all the research. How many of them start with, "Did you ever notice . . ."? Frequently, those are the observations that get laughs. Yes, we noticed, but never quite thought of it that way. You can do that in your writing, and that makes the character memorable. Seinfeld is a perfect example: The Close Talker, The Fast Talker, etc.
For writers, it's not about saying something new; it's about saying something in a new way. I've got the noticing part down, which I would say is at least twenty percent of the battle. Now I have to work on writing about them in such a way that readers see it through new eyes - mine (or rather, my character's). At this point, I've almost done too much noticing and my poor characters are being experimented with in every which way. I have to make them interesting, but not twitchy. So do you think a main character who blinks constantly when she's nervous, clears her throat, has a lisp, stamps her foot, says "Ha!" all the time, drums her fingers, chews her nails, talks with her hands, talks loudly, and invades personal space is too much?
What about a character that pushes up her glasses with her middle finger?
A few well thought out quirks go a long way. I'll have to just pick one or two, and go from there.
I think characters should have more influence from things like generation, family, work as opposed to personal quirks. But the few that you pick are important. Subtle, telling, individual.
I'm not sure what I'm giving Cassidy for my upcoming NaNoWriMo novel that I'm starting in November. I want her to be likable, so it will be subtle, or funny. I'll keep looking and I'm sure I'll notice something in the next few days that will be perfect. I'll let you know after November.