During an interview with Collider, Kevin Feige talked about why Josh Brolin makes the perfect Thanos in the Marvel universe. He also revealed that Brolin didn't only voice the character, but they also shot the actor on set. Regarding the casting of the universal villain, Feige said,
“We were looking at a wish list of ‘Wouldn’t it be great if’ names, and his name was on it. You look at his face and the performances he gives — he could be Thanos without any effects. He has that kind of face and that kind of gravitas to it.”
He then says that Brolin was an easy sell on the project, and that they wanted him to be more than just the voice.
“We reached out to him and it was one of those things that does not happen all the time, but when it does it’s very nice, where he was totally intrigued. We ran it by [Guardians of the Galaxy director James Gunn] who loved it, ran it by [Joss Whedon] who loved it because Thanos is in this universe because of Avengers. Then we shot him and recorded for it. We wanted somebody to be more than just the voice. Josh did the performance as well.”
I think we can all agree that Brolin is the most badass actor choice they could have made for Thanos. I can't wait to see how this character is portrayed in Guardians of the Galaxy!