Diesel posted a photograph of himself to his Facebook page earlier today--posing with a bust of Groot from the film adaptation of Marvel's Guardians of the Galaxy, and with the film's logo in the background.
This isn't the first time Diesel has got fans talking by posting a Marvel-centric image on his Facebook page; the first time, it was a fan-made photoshop of himself with the caption "True Vision," which popped up shortly after rumors started that Marvel might be interested in the Fast & Furious star for the role of the Android Avenger.
Not long after that, he piled on, posting a photo of himself in front of an oversized Avengers comic book cover, and confirming for fans that he was talking to Marvel Studios about...something.
Then San Diego Comic Con International happened and while many expected an official announcement during that weekend, one didn't come. Shortly thereafter, Diesel posted the most specific teaser yet--a comic book image of Groot. It wasn't long after that he began talking about the role as though it were a done deal--as did Guardians cast members--although both director James Gunn and Feige have said nothing was formal yet.
It's possible that, following the death of co-star Paul Walker and the subsequent filming delays to the latest Fast & Furious movie, Diesel's schedule has opened up somewhat and the scheduling conflicts that have been cited as stumbling blocks with Marvel in the past have been partially cleared. It's equally likely that Marvel simply wasn't ready to confirm for other reasons, and Diesel has already finished filming--especially when you read comments from the film's director.
Gunn re-shared the image on Facebook and WhoSay, adding, “Vin Diesel is a super awesome dude, I can't tell you how much I like this guy,” and, “I hope this [photo] puts to rest all the silly Thanos-Vin stuff which people just seemed to make up out of nothing.”
Regarding why he wasn't officially announced before, Gunn added, “He said he wanted to do it and we said we were talking to him. Everything else was people guessing and thinking they knew what was going to happen.”
“This was the bust we used for reference when shooting. It has changed slightly from this since we started animating him," the director added of the bust itself.
Asked whether this constituted a confirmation of Diesel as Groot, finally, Gunn said, "I hope so because I'd hate to think I spent all this time recording him for no reason."