How do genre overlap each other?

How do genre overlap each other?
In music, it's pretty easy to overlap genres. Taylor Swift, for example, is a country-pop crossover. She uses a rock and roll beat and less twang than straight country music, but she also relies on typical country music instruments and song topics.
In movies and books, it's a little harder. However, a classic example is The Princess Bride because it uses witty humor, romance, and adventure. It can't be categorized as a comedy, chick-flick, or action film because it uses a little bit of each.
A general rule: take a few characteristics of each genre you are combining and put them together, you'll be in the overlap.