Phoenix admits he certainly had his reservations at first. Speaking with TotalFilm, the actor says it took him a while to commit, explaining why he was hesitant:

“It took me a while [to commit]. Now, when I look back, I don’t understand why… There was a lot of fear, yeah. But I always say there’s motivating fear and debilitating fear. There’s the fear where you cannot make a fucking step, and there’s the kind where it’s like, ‘OK, what do we do? That’s not good enough.’ And you’re digging deeper and deeper. I love that kind of fear. It guides us, makes us work harder.”

There was another reason Phoenix was hesitant, and it has to do with the way superhero or comic book movies are usually told:

“I think oftentimes, in these movies, we have these simplified, reductive archetypes, and that allows for the audience to be distant from the character, just like we would do in real life, where it’s easy to label somebody as evil, and therefore say, ‘Well, I’m not that.’”

Read more at Collider.