Way back in December, Vin Diesel was officially announced to be voicing Groot in Guardians of the Galaxy. This had been rumored, and pretty much stated by Vin Diesel himself, months before, but became100% official in December.
In case you don't know, Groot is a walking tree. He's not quite like an Ent, but he is sentient. Groot is primarily known for only saying his signature catch phrase "I am Groot!". Allegedly when he repeats this phrase, it can mean any number of things in his native language, but to everyone else he's just repeating his name.
While it's unclear whether or not that will factor into Guardians of the Galaxy this August, but let's assume for a second that they will keep that from the comics. While Vin Diesel isn't the biggest name in Hollywood, I would say a majority of moviegoers know who he is. Mostly due to the increasingly successful Fast and Furious franchise, but with some help from the not as successful (but well liked) Riddick franchise, Vin Diesel is becoming a bigger name each year.
So with Vin Diesel becoming even more and more popular, would it be wasting potential to just use his voice for one line with different inflections? On the other hand, the Marvel Cinematic Universe is on course to be the most successful franchise of all time, as it's undoubtedly going to be the highest grossing franchise worldwide within the next few years. The MCU is showing no signs of slowing down, so does it necessarily need to take advantage of every actor they can?
The biggest casting question in the Marvel Cinematic Universe right now seems to be "Who is playing Thanos?!" There are a number of rumors and speculation. I've read many articles saying Vin Diesel and Benecio Del Toro are signed on for multiple characters, but I can't seem to find any confirmation on that. Others speculate that an unknown actor may be playing Thanos, as he might just be computer animated. Personally, from what we know so far, I would say Thanos is going to be a huge part of the Marvel Cinematic Universe. I have my own theories (that definitely dictate their own post), but Thanos has pretty much been behind the scenes from the beginning. I'm not saying Thanos has been pulling all the strings, but he's been there, watching. The Chitauri invasion in The Avengers was undoubtedly part of his plan, and I bet it goes further than that.
If you don't already know, Thanos is the guy who turns his head and smirks at the end of The Avengers! Thanos is a super-powered cosmic being. He is obsessed with death, and generally considered one of the biggest threats in the galaxy. Most likely, Thanos is going to appear in Guardians of the Galaxy, and then be the focal point of Avengers 3. It's safe to assume that Thanos is going to be a big deal, for more than just one movie too. Imagine if they hadn't cast Tom Hiddleston as Loki? There are plenty of other actors who would do a wonderful job, but Loki is just so iconic and memorable. It would be foolish to not cast a memorable actor in this role, unless of course he's going to be all CGI. That being said, I didn't know who Tom Hiddleston was before Thor, and I didn't really care to. It just goes to show that they don't need a major actor in a role to make it iconic.
Vin Diesel playing Thanos would work great. He can be menacing, he's physically built for it, and he's popular enough that he might actually bring new fans into the series (if there's anybody left who isn't watching them). We all know Vin Diesel can handle voice acting (Iron Giant), so it wouldn't be awful if he just played Groot, but he seems like a big actor to record the same line over and over again.
Guardians of the Galaxy opens on August 1st and stars Chris Pratt, Vin Diesel, Zoe Saldana, Dave Bautista, and Bradley Cooper.