It's hard for me to say whether I "need" to write or whether I simply need the time to do it. When things are normal and life is quiet, I generally write in the morning. As soon as I sit down to do it, the words come. But when something interrupts my schedule (usually Life with a capital "L"), writing moves to the back burner. Even if I try to snatch a few minutes' writing time in the swirl of events, nothing good results. It's like my brain shuts down in self-defense, leaving me resources to deal with reality.
On the other hand, when writing is foremost in my life, when I have a deadline or when the book is going well and I want to see how it ends, I find it hard to care about what's going on outside my head. A group of my friends are having lunch? I'll catch up with them next month. My husband's headed out to shop? I'll make him a list of what I want. At that point, all I can focus on is getting back to my "people" to help them figure out what's happening to them and why.
Another oddity is that one book doesn't usually intrude on another. I have three series going, but my mind is willing to focus on one at a time. Simon from the Tudor series doesn't knock at the door of my mind and insist on telling his next adventure. He's apparently willing to wait until I finish telling Loser's story.
I picture little compartments with labels like "Book #3 Dead Detective" or "Alumni Association" and even "worry about ---". The beauty of it is that only one door is open at a time.
That's a good thing. If it weren't the case, I'm pretty sure I'd be stark, raving, mad by now.