Mark Ruffalo has a reputation among members of the Avengers cast.
He's not the only actor who's had issues with letting spoilers slip, of course. Tom Holland, Chris Pratt, and others have had their issues, but even with that in mind it's Ruffalo who's become the actor best known for saying things he shouldn't when promoting the Marvel Cinematic Universe. He speaks a little too freely in interviews, once left his phone on during a screening of Thor: Ragnarok, and even spoiled a good chunk of the ending of Infinity War nearly a year before that film came out. He's a Marvel PR headache, but at least he's good-natured about it.
To prove just how good-natured he is about his reputation as the Most Spoilery Avenger, Ruffalo turned it into an extended joke for his Friday night appearance on The Tonight Show Starring Jimmy Fallon. Hours before the show was set to air on Friday, Ruffalo sent out a tweet teasing that he'd let something slip on the show, and begging Fallon to cut it from the broadcast.
Fallon was game to play along on his own Twitter account, further setting up promotion for the show that night.
Even Ruffalo's Avengers 4 bosses, Joe and Anthony Russo, got in on the act.
Then it was time for the show to air, and during his interview with Ruffalo, Fallon brought up Avengers 4, asking if Ruffalo could tell him anything. Ruffalo did some of his finest Bruce Banner hand-wringing and noted that he's been "put under surveillance" by Marvel for some of his past slip-ups. Fallon persisted, and asked if Ruffalo could at least reveal the title.
"All right, I think I can probably tell you that," Ruffalo said, and because he's a very good actor, for a moment it felt like he might really be letting something slip. Then, of course, as soon as he started speaking, he was bleeped and a black box was placed over his mouth. To further drive the bit home, Fallon asked for even more spoilers, and Ruffalo kept talking -- his words bleeped the entire time -- until he was climbing on furniture and pulling pieces out of Fallon's miniature cityscape set. Fallon ended the bit by beggin Ruffalo to stop, shouting "That's enough spoilers!"
So it was all a semi-elaborate joke for the sake of a late-night comedy show, and Ruffalo was actually there to promote a young voter registration initiative in the next segment, but his reputation is such that even we bought it for a minute. If anyone in the cast is likely to just give the title away in front of a live studio audience that could easily then start tweeting about it, it's him. At any rate, the real Avengers 4 title remains, for the moment, officially still a mystery.