I will, occasionally, insert things into my novels strictly to amuse myself. My rule is that I can slide these private jokes in only if they don’t interrupt the story.
In Shades of Milk and Honey, I managed to fit Doctor Who into the Regency.
Oh, yes. I’m not kidding about this. He’s easy to spot if you know he’s there, but if you aren’t looking for him, he blends right in. In Glamour in Glass I slipped another incarnation of the doctor into the text.
Can you spot him?