Your character feels like a real person and you have imaginary conversations with them... They tell you stuff about themselves you didn't know. Like they prefer cats.
"the wedding was charming if a little gauche; there's only so far new money goes.... it must have been her fault his heart gave out"
You know you're a writer when you keep a pen and pad of paper on your bedside table so if you get an idea at one in the morning while trying to sleep you can easily write it down.
You know you're a writer when you stare at strangers for probably a little too long, seeing them as potential character inspiration while mentally contriving a backstory for them.
You know you're a writer when you spend half the day procrastinating thinking up random plot or character ideas.
Dumbledore: "Now, it's great that you've been saving the school and all Harry, but unfortunately your grades have been a tad low, and, well... perhaps Gandalf could explain it better... hit it, Gandalf!
