How did Gaiman use humor, fantasy, and suspense to describe Bod and Scarlett's encounter with the Indigo Man?

In chapter 2 of The Graveyard Book by Neil Gaiman, Bod and Scarlett travel into the Frobisher's mausoleum in search of ancient barrows. In the process, they meet a frightening figure called the Indigo Man, who threatens them with bodily harm unless they leave.
Bod narrates their descent into the barrows because Scarlett can't see in the dark. It's already been established that Scarlett can't see or interact with ghosts the way Bod can, so it's a surprise to both of the them when they meet the Indigo Man because he's as vivid and frightening to her as he is to Bod.
Though it's a frightening scene because of how threatening he is, there's also a bit of humor to the encounter with the Indigo Man. His threats and actions are a little overdramatic, especially in comparison to what comes next—he lets out "yodeling screams" and threatens to eat the children's livers, but, once Bod realizes that he must not be real because Scarlett can see him, they start discussing him as if he were a scarecrow. The fact that a frightening creature like the Indigo Man, who is implied to have caused the death of somebody who ventured into the barrows at some point, is calmly discussed by two children, is pretty funny despite the potential scariness of the scene.
The fantasy element comes in because the Indigo Man is imaginary. Generally, an imaginary creature is nothing to be afraid of because you can control it with your own imagination. In this case, though, the imaginary creature isn't imagined by either Bod or Scarlett, so they have to figure out what it is and how to stop it on their own. The Graveyard Book concerns many fantastical creatures, such as ghosts and night-gaunts, and the encounter with the Indigo Man is just one of them.
Finally, though the encounter with the Indigo Man ends favorably for Bod and Scarlett, it's still a very suspenseful scene. The Indigo Man is unfamiliar to Bod, who knows most of the inhabitants of the graveyard, and does not respond favorably when he tells him that he has the Freedom of the Graveyard. Though the children overcome the Indigo Man by refusing to be afraid of him and acknowledging that he's imaginary, that raises an important question: who is imagining him? As it turns out, there is something more frightening beyond the Indigo Man, heightening the suspense even further.