I am a completely horizontal author….
I can’t think unless I’m lying down, either in bed or stretched on a couch and with a cigarette or coffee handy. I’ve got to be puffing and sipping (Truman Capote)
George Plimpton, The Writers’ Chapbook (Viking 1989) p. 52
And as it happens The Millions has a nice article on the same subject: http://www.themillions.com/2014/07/how-to-be-james-joyce-or-the-habits-of-great-writers.htm