This answer might fit into the "wish I'd never asked" category, but here goes.
My novel started with the simple idea of one particular character, based entirely on a former boss I had. It was very easy to give him a role because the only interests that the original had were golf and football. I also had a whole load of situations that I knew I could turn into a form of sketches, but I didn't really have a plot.
So I started writing some of the "sketches" and involved a few other characters. I'd got a fair way through, but still wasn't quite sure where it was going. What set it on the right path was the introduction of a new character, based on a Glaswegian girl I knew from Oxford. Funnily enough, I was unsure about using this character at first, but she turned out to be one of the most important characters in the book.
In the end, I was able to concoct something that involved use of characters that had appeared earlier in the novel and I managed to weave in a series of misunderstandings that made the plot work. So the plot rather evolved as I was writing the book.