You spell certain words so as not to wake up your character.
You spend the day working alone, and you have two major fights with other people… in your head.
You read your early-published stuff and want to throttle your younger self.
You can’t write the story on deadline because this other story is SO LOUD.
You were depressed for two weeks after one of your characters died.
You worry about the ethics of your two most-messed-up characters having kids… then dip into the kids’ time to make sure they’re okay.
You check on your friends in several worlds, as you’re falling asleep.
You have to tell yourself not to make your trip to Walmart a “better story” by adding an alien attack in the produce section, when you tell your spouse you couldn’t find his/her brand of shampoo.
Your characters harass you, late at night.
You have to remind yourself not to stay in your fictional worlds – because they’re more interesting and more fun than the real one.
You have a feeling if you didn’t write stories, you’d be in a straight jacket.
You forget you’re driving and end up 100 miles away, because you were working out a thorny plot problem.
You worry because you discover a movie uses something like your book’s plot, and you worry people will think you took it from the movie… which didn’t come out until the book was published.
You think up ten ways of murdering people with a coffee cup, first thing in the morning.
You start asking your husband to call your character then realize this is not possible.
You’re not absolutely sure how many kids you have, because you made one up years ago. What if the universes get confused?
You often yell at the sky (or the ceiling) “What made You think that was a good plot twist?”
But the number one reason you might be a writer is that you’re reading this instead of writing…
Now go write (but leave us your contributions.)
Mad Genius Club 
I’m trying!
I’m mulling what exactly this scene needs to pop – because it is crucial, but tiny.
LOL!
At any given time your office bookshelf looks as if it belongs to: a European historian, a chemist, an aircraft mechanic, an English-lit teacher, or a biologist. Or “yes.”
You realize that you’ve diagrammed alien DNA on a napkin at lunch, including gene replacements. While you were supposed to be listening to the gal talking at the front of the room.
You are totally surprised when one of your characters gets married because, “He’s supposed to wait until book Four, dang it!”
You critique the pacing and lack of well-developed protagonists in ongoing political scandals/Hollywood divorces/natural disasters/annual family Thanksgiving Day squabble.
You drive your spousal unit crazy by announcing plot developments as or before they happen in his/her favorite drama.
He/She responds by yelling, “SPOILERS!” and you go, “I’ve never seen this before.”
M
You go down Sarah’s list muttering, “Check. Check. Yes. Wrong gender. Yes. Check. Check…”
M
I get about 90% of them – DH has learned to take it in stride.
I ONLY comment on the really obvious, egregious ones.
Oh yes!
You fall asleep with the beginning narration of a story running through your head and you wake up reaching for a pen instead of the alarm clock.
You hear tornado sirens and your first thought is making sure that your WIP is backed-up online.
You see a sexy girl/guy and think about which of your characters would hit on her/him.
You’ve turned down an invitation to a night out by saying, “I have to stay home and plan a murder”.
And the person you’re talking to doesn’t bat an eye.
You’ve listened in to other people’s conversations and mentally rewritten their dialogue to sound more natural.
YES! All of this. And when wild fires look likely to push you out of your house, you carry your stuff on flash drive on you at all times — even when you’re sleeping 😛
. . . the first thing to go into your Trouble Bag is your backup hard-drive.
. . . you’ve ever clutched that bag and muttered, “If you go, I go.” (In my defense, it was dissertation material and fiction work. And I was really, really tired.)
…your characters hang around making disgruntled, impatient noises because you aren’t writing THEIR story. (This does not include the Narrator, who is hoping I’ll never finish because IT’S ALL SO HORRIBLE AND TRAGIC.)
Oh, yes. Classic case.
…you are recognized on-sight by the folks who work the History sections at your local bookstores… *ALL* of them….
and they hide when they see you coming 😉
My universes/worlds are pretty damn dangerous. Not sure a fat old boy could survive.
You keep arguing with your character because she insists on telling the story in first person, when it’d be easier to tell the story in third.
I fought DST tooth and nail over this.
You end up with 3 different WIP open (Much easier on laptop. This situation can be total nightmare if resorting to paper…dang it I lost the scene/dialogue I NEED right NOW before I forget what I wanted to add…) as well as 3+ research sources for each WIP (at least one of which is overdue) covering your desk, and every other flat surface in a 5 foot radius from your workstation including the floor. Then panic when the phone rings for not only is your “writing groove” disrupted, you also can’t find the noisy contraption that is hiding somewhere under the “pulp plethora”. (Or in my case, the phone can also be under our cat’s tummy. Wouldn’t be so bad if he hadn’t learned to tune out the ringing…)
You have learned to time your writing to the swishing of a cat’s tail or the 2 minute “all is quiet” intervals without a child starting a fight with a sibling or trying to find you.
Friends and acquaintances see you sneaking up on them with a stack of papers and remember that they can’t possibility ‘beta’ for you since they must run home to bathe their cat, pluck their nose hairs, drink turpentine, etc.