I think every writer develops tics - habits that don't (usually) affect the content of their writing, but become an integral part of the process. These become necessary ritual, prelude, part and parcel - a mental signal that writing is afoot. Here are some of mine:
Before starting a story, I have to choose a font. I have a group of maybe six fonts I use commonly, and which font I'll choose depends on the "feel" of a story. A Trebuchet MS story is not an Arial story. I've written some epistolary stories, and I used to insist on using Monotype Corsiva (it looks like good handwriting).
I always stop writing (and hence start writing) in the middle of a sentence. Yes, this means occasionally I come back to it and I don't know where I was going or decide the sentence should go in another direction.
It has to be blazingly bright. I don't like a dim space; I don't even like a semi-lit space. I have no fewer than three lamps around my desk, and at night, all of them are on.
I (almost) never close a story file while in progress. Word remains open all the time.
If I'm writing while multi-tasking (that is, during the day while working), I stop at set inches down the page to swap tasks.
This doesn't get into tics that actually affect the content, such as the fact that I generally won't start writing until I have a title.
So ... if you are a writer or know a writer, tell me about your / their tics.