Every teacher has a favourite student, just like every director will have a favourite character. With Tim Burton, that favourite is Jack Skellington, from The Nightmare before Christmas.
So which that in mind it makes sense he’d try to include Jack in his other movies. In fact, the character makes cameos in Burton-related flicks including James in the Giant Peach, Coraline, Alice in Wonderland, The Princess and the Frog, and Sleepy Hollow.
Even cooler, before the Nightmare Before Christmas even made it to the big screen, Burton managed to sneak Jack into the 1988 movie, Beetlejuice. His head can be seen on top of Beetlejuice's carnival hat.
This clever cameo in his films softly links them all together into one universe, almost being haunted by Jack himself.