In a hero action beginning, the hero is onstage, doing something active and interesting related to the launching of the core story (it need not involve explosions and car chases, but it certainly can).
Groundhog Day begins with Phil Connors onstage giving a (sarcastic) weather report. WALL-E begins with WALL-E onstage doing his daily routine of garbage collecting and compacting. Juno begins with Juno walking through the neighborhood, drinking SunnyD, on her way to the corner store to buy a pregnancy test. Nearly every James Bond story begins with 007 performing some amazing derring-do. What About Bob? begins with Bob going through his neurotic morning rituals.
The hero action beginning is the other most common way to begin a story. Only the rarest of story ideas can’t manage a hero action beginning. Unless your hero is catatonic or incarcerated in a hole or the like, I’m certain you can come up with something interesting for him to do at the start of the novel.
But remember to ask yourself how much of a stretch is it to show that action. And would a prologue (or some other approach) help you more than a hero action beginning? Now you’re thinking strategically about your story—an excellent and essential thing to do.
Some books lend themselves naturally to a hero action beginning. If the protagonist is a superhero when the story begins, you can start the novel by having her save the earth. If he’s a football player, show him on the field in a big game. If she’s a karate champion, show her winning a tournament.
But if your hero isn’t a hero yet or isn’t yet in a position to show it—or if you simply prefer to establish your villain and time bomb in a prologue—perhaps the hero action beginning isn’t right for your book. Mulan begins with a prologue because the protagonist isn’t yet in any kind of heroic capacity. Mulan is feeding chickens on the family farm—not necessarily an interesting introduction. The writers could’ve invented a way for her to be heroic at the outset, but they chose not to, and I agree with their choice.
Don’t force a hero action beginning. We all could make up something for our heroes to do as the book begins. But if it feels like a stretch or a cliché, choose another approach.