Rowling's whole story of Harry Potter's inception in 1990 is so good:
"I was travelling back to London on my own on a crowded train, and the idea for Harry Potter simply fell into my head. I had been writing almost continuously since the age of 6 but I had never been so excited about an idea before. To my immense frustration, I didn't have a pen that worked, and I was too shy to ask anybody if I could borrow one . . .
I did not have a functioning pen with me, but I do think that this was probably a good thing. I simply sat and thought, for four (delayed train) hours, while all the details bubbled up in my brain, and this scrawny, black-haired, bespectacled boy who didn't know he was a wizard became more and more real to me.
Perhaps, if I had slowed down the ideas to capture them on paper, I might have stifled some of them (although sometimes I do wonder, idly, how much of what I imagined on that journey I had forgotten by the time I actually got my hands on a pen). I began to write Philosopher's Stone that very evening . . . "