Most of the time when I’m writing the idea comes first. There’s an idiom to twist, a play on words, some jargon that needs puncturing … But sometimes you just get an image in your head and you go backwards.
Take for example this Trojan warrior in an office. Why is he there? What does he want? What’s his viewpoint?
The Trojan horse is a natural gag here, but I don’t think I’d have come upon it if I hadn’t imagined this oddball scene first.