Regardless of genre, humour is always welcome. But a moment of levity is especially welcome when it distracts the reader away from a serious backdrop. A single, short, well-delivered line can make your audience forgive ten chapters of demure tragedy. Humour that pokes fun at your characters or some element of your story can be an indirect way of exposing some truth. It also reminds the audience they’re here to enjoy themselves, and shows them that you don’t take yourself too seriously.