Thanos was first seen at the end of 2012's "Avengers Assemble", played by Damion Poitier. He was real and made up with prosthetic make-up, but it's been rumoured that Brolin's full incarnation of the character will be CGI with only Brolin's voice being used to bring the rest of the villain to life.
Personally I can't fricken wait to hear his voice as Thanos. Can't wait to see Thanos himself come to full life on the big screen in full form. Brolin will rock that role with his awesome voice.