I have had a long standing love for Doctor Who, dating back to middle school when I was watching Tom Baker episodes. The nice thing about a time traveler is that he can turn up anywhere so... in each of my historical fantasy novels—Shades of Milk and Honey, Glamour in Glass, and Without a Summer—I’ve inserted an unspoken cameo from the Doctor.
My rule is that I can slide these private jokes in only if they don’t interrupt the story.
[Where to find the Doctor in Shades of Milk and Honey, Glamour in Glass, and Without a Summer]