In Keith Richards autobiography, Life, he talks in detail about a number of key elements in the Rolling Stone's creative process. Including creating a unique sound by plugging an acoustic guitar into a cassette player. His best story is about the creation of the hit song "Satisfaction" as he told it to Terry Gross on NPR's "Fresh Air".
"I go to bed as usual with my guitar, and I wake up the next morning,
and I see that the tape is run to the very end, And I think, 'Well, I didn't do anything. Maybe I hit a button
when I was asleep.' So I put it back to the beginning and pushed play
and there, in some sort of ghostly version, is [the opening lines to
'Satisfaction'. It was a whole verse of it. And after that, there's 40
minutes of me snoring. But there's the song in its embryo, and I
actually dreamt the damned thing."
He goes on to explain how it was a skeleton that Jagger built on to make great.
It's a very good example of how creativity can strike at any moment and it pays to have something with you to get the idea down, but importantly it's critical to understand how an initial idea can be built upon and strengthened.