Mark Ruffalo is hard at work on The Avengers: Age of Ultron, his second turn as the Incredible Hulk. But what does the future hold beyond the Avengers sequel?

The actor has already made it clear he doesn't see a Planet Hulk film working out. But is he closed off to a solo Hulk movie entirely? Absolutely not — but it comes with its share of challenges.

"If we could find a story that's compelling enough to hold people's interest for the two hours — that is essential," Ruffalo tells Indiewire. "It's a tough nut to crack, and I see it as a tough nut to crack. I think it's tougher than the other superheroes because you have a guy who essentially doesn't want to be there doing the thing that you want him to do more than anything. So it can be frustrating as an audience member. It could become a little mirthless."

Not that Ruffalo and the rest of the Marvel team aren't trying to add some mirth to the character, mind you.

"With the new technology there's a lot to do in Hulk-land that we haven't done yet that is really exciting. So you could have a balance of the two," he says. "And I've been finding this relationship between Banner and the Hulk and Hulk and Banner is equally compelling -- it's been explored in the comics, but never in the movies. It's always been Banner's relationship to this sort of lump, this unknowable very two-dimensional thing. But there's also something really interesting about Hulk's relationship with Banner. The only thing that scares Hulk is Banner. It's not some bigger, scarier, huger thing — it's this frail man. And it terrifies him and it angers him."

"It's such an interesting relationship that no one has cracked," he continues. "I feel like that would be an interesting place for us to go. That might hold people's attention."

For now, Ruffalo makes his next Hulk appearance in The Avengers: Age of Ultron, in theaters on May 1, 2015.