Watch Jimmy Fallon Tease Biggest ‘Avengers 4’ Spoiler Ever From Mark Ruffalo

Notoriously loose-lipped Hulk star Mark Ruffalo will drop an Avengers 4 spoiler on tonight's episode of The Tonight Show, according to host Jimmy Fallon.

Ruffalo could officially announce the official title, which directors Anthony and Joe Russo dubbed a "spoiler." Marvel Studios chief Kevin Feige said promotion on the Avengers: Infinity War sequel would gear up once the Brie Larson-led Captain Marvel had gotten itself out in front of the world.

Weeks removed from the first Captain Marvel trailer, Marvel is gearing up for the mysterious Avengers 4, which in recent weeks headed back in front of cameras for a second round of reshoots after additional photography commenced at the start of the year.

Ruffalo makes his return as Bruce Banner-slash-Hulk after Banner's angry green alter ego was a no-show following a brutal defeat at the hands of Thanos (Joah Brolin).

According to Ruffalo, the second round of reshoots were to finish the film — Marvel's biggest ever.

"Well we're doing reshoots starting in September," Ruffalo said. "And then we'll go back into the international tour, we'll go on tour. Then you guys will get the second installment. Which we don't even know what it's gonna be yet. We're not just doing reshoots, we're going to finish the movie, which we really didn't get to finish totally when we left it last year," he told the Marvelists Podcast.

Because the project is in a state of flux — and the Russo brothers are notorious for playing their cards close to the chest — even its stars aren't quite sure which way the story is headed or how this first chapter in the ten-year Marvel Cinematic Universe story will end.

"I don't even know that they really know exactly," Ruffalo said of the Russos and screenwriters Christopher Markus and Stephen McFeely.

"Some of it is happening while we're there. It's pretty amazing. And we'll shoot some stuff and a few days later come back and reshoot it cause we wanna take it in another direction. It's a very living organism, even as we approach it being a locked picture, we're still working on it."

Avengers 4 opens May 3, 2019.