It seems obvious--a protagonist should be strong, propelling his or her action forward by decisions and actions born from motives and goals that make sense.
Yet writing a passive protagonist seems to be one of the insidious traps that writers-in-training (and even writers-with-lots-more-experience) fall into.
Recently, I read a YA novel in which the protagonist pretty much waited around for others to tell her what to do. Mind you, it wasn't that blatant. She had strong emotions, so she didn't seem passive. But when it came right down to it, she didn't really do anything, so I never really cheered for her.
She allowed a tough-girl character to lead her around, and didn't demand answers when the girl refused to answer simple questions (such as, "Where are we going?").
She followed her (jerky) boyfriend around and didn't argue when he had questionable behavior (like getting drunk and sort of forgetting about her at a party).
She had hard questions she needed to ask certain people, and she never really came out and ASKED them.
You're probably nodding your head as you're reading this, but you'd be surprised how easy it is to write protagonists this way. We make things *happen* to them, which makes us think that things are all kinds of exciting. Except, when things are happening to our protagonist without any definitive action from him, we've got a passive protagonist.
So, what to do?
Well, take a hard look at important scenes in your story--major plot points, revelations, twists, etc. What is your protagonist DOING? Following someone else's lead? Running away? Waiting for instructions? Asking for help? Engaging in long, mind-numbing internal dialogue?
Or is he DOING something? Is he asking hard questions? Demanding answers? Is he making a hard decision and sticking to it? Running into the thick of the fray? Killing someone? Saving someone's life? Saying "no" when everyone expects him to say "yes"? Or visa versa?
A protagonist doesn't need to be a superhero (unless, yanno, you've written a superhero story), and, in fact, won't be believable if you try to make him into one. But he's got to be bigger than life in the sense that he propels the action--makes the choices--brings on his destiny. Nobody wants to read the story of a mamby-pamby wallflower. (And even if your story is about a wallflower who succeeds against all odds, you can still make him ACTIVE instead of PASSIVE--by giving him the actions that propel the story, instead of following the supporting characters and all their shenanigans.)
So revisit your protagonist! Make sure you've written a character for whom we WANT to cheer. Who draws us in and makes us think, "Why don't I think of things like that?" Not someone who climbs down the fire escape because her best friend told her to.
Got it? Now go make sure your main character is positively kick-butt!