Alright, so he never actually said the words "I am Groot"--he's probably contractually obliged not to say those words anyway, lest he spoil something--but when asked about his role as Groot in the forthcoming James Gunn adaptation of Marvel's Guardians of the Galaxy, Vin Diesel answered directly and without qualification that he may not actually BE Groot.
"I’m an actor. I can do whatever the … I want," he told the Los Angeles Times's Hero Complex blog. "As an actor, not everything has to be the most obvious choice. And sometimes, the best thing you can do — as far as Steven Spielberg and his advice — is to defy expectations. So if everybody thinks you’re going to go for this one thing and you flip it entirely and go for the strangest Marvel character, it’s interesting. And when something is interesting, it’s inspiring."
As to why he posted the image of Groot to his Facebook account this weekend, seemingly blowing the secret just hours after Marvel declined to identify his role at the D23 Expo presentation for the film, the actor said, "I get inspired by things. So if somebody sends me a book of conceptual art that’s the best character design work I’ve ever seen — I’m not kidding, I see that and I want to be a little cryptic and I want to say something positive. Even if it’s as cryptic as an old, dated image of a character."
It does sound, though, like Groot might be as small a voice acting role in the film as fans might expect from the comics. The actor acknowledged that filming is about half over for the film, and that part of the appeal of Groot is that it wouldn't be a huge commitment of time.
"That’s the thing — I’m so busy. The audience wanted Vin and Marvel, Vin and Marvel," Diesel said. "But I’m too busy to do a six-month role. So what Marvel came up with was really interesting. … I am watching a social wave influence and in some ways guide their thinking."