One of the most common forms is dramatic action, which engages the reader into wondering what the consequences of the action will be. This particular form has been recommended from the earliest days, stemming from Aristotle, and the widely used term in medias res stems from the Roman Empire. But action is not, in itself, a hook, without the reader's wondering what will happen next, or what caused the actions to occur. Overly dramatic openings may leave the reader indifferent because the characters acting or being acted on are non-entities; even murder of a faceless character may not engage interest.
The use of action as the hook, and the advice to so use it, is so wide-spread as to sometimes lead to the use of the term to mean an action opening, but other things can be used for narrative hooks, such mysterious settings, or engaging characters, or even a thematic statement, as with Jane Austen's opening line, "It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."
When a story does not lend itself to a good hook when it is laid out linearly, the writer may tell the story out of order to engage the reader's interest. The story may begin with a dramatic moment and, once the reader is curious, flashback to the history necessary to understand it. Or it may be told as a story-within-a-story, with the narrator in the frame story telling the story to answer the curiosity of his listeners, or by warning them that the story began in an ordinary seeming way, but they must follow it to understand latter actions.