Which one would you want to read more about?
A: A villain enters the room, their sword drawn. Our hero is cornered. The villain attacks, the hero pulls out his/her weapon and fights back.
B: A villain enters the room, their sword drawn. Our hero notices the villain entered, draws their weapon and attacks.
Which “hero” is more interesting? I’ll bet you picked option “B.” I did.
Hero b is more active than hero a, and it’s human nature to prefer someone who takes action. But hero b is still reacting.
To be honest, it’s difficult to think of a situation where a non-villain is 100% active, not reacting against something an antagonist did. An example I’ve heard cited often is Indiana Jones in the first scene of Raiders of the Lost Ark. The audience sees him go after the treasure without having any reason other than his own motivation. I haven’t seen the movie in some time, so I can’t recall it’s explained later that he’s reacting to something, but, for those first few scenes, he’s a 100% active character.
But even some of the most beloved characters react. Breaking Bad’s Walter White reacted against his lower-middle class lifestyle, getting cancer, his wife’s pregnancy, and who knows what else when he decided to cook meth. Fullmetal Alchemist’s Roy Mustang decided he wanted to rule Amestris after participating in the tragedy that was the Ishvalan war. While most would label these character’s as active, and most of what they do in their respective tales is whatever gets them closer to their goal (within reason, for Roy at least), they still reacted.
So, how is this any different than a superhero that doesn’t come out to play unless the villain does something first?
Well, because Walter, Roy, and hero b were active in their reacting. While unnamed superhero (I truly don’t have any particular one in my head) and hero a were reactive to the point you feel that without a villain, it would be a story about them going to school or work or something.
And I think there’s the difference. Even without villains, Walter and Roy (well, debatable that Walter himself is a villain, but that’s another argument) would do something. They both have goals they work towards. The antagonists and everything else thrown in their way just makes the trip more interesting.
To take it to a scene-by-scene level, we’ll go back to the example. Hero A and Hero B are reacting to something another character did. However, Hero A takes very little action. They literally don’t do anything until they have their choice is die or lift the sword. Hero A sees something bad happens and decides to act against it rather than react. While, for the sake of simplicity, I made Hero B choose a standard action, to fight back, what if a Hero C decided to join the villain or spy or to escape later. What if Hero D is a merchant and distracts the villain with some ridiculous sales pitch then throw salt into the villain’s eyes. What if Hero E decides to surrender into enemy hands or runs away from the fight. All of these are actions a character could take and all are more interesting than Hero A’s choice to stand there, even Hero B’s fairly standard reaction is still more interesting and could become more so with more detail (are they a good fighter or bad, do they use a sword or axe, did they do it in defense of their own life, money, someone else’s life?). It tells the reader a lot more about the character and makes them seem more alive because they are not acting like a prop moved whenever the plot pushes them, but something alive because they take an action and prove they have a mind of their own.
And I think that’s what it comes down too. Characters seem more alive when they take actions because it seems like there is thought behind what they did. So, even if reacting to something, as long as they are reacting with an action and look like they’re making a decision, you’re probably set. But I wouldn’t use this as a crutch. Do your best to make your characters active, but you don’t have to kill yourself over it. Just make sure your characters are making decisions when they still have options other than fight or die and that the character would have a story, even if an antagonist never showed up.