It seems a bit strange that Josh Brolin's casting as Thanos would just now be completed, considering the character has been confirmed to be kind of extremely important to the plot of Guardians of the Galaxy which will be coming out this August. With only two months until the film's release, unless Brolin has mysteriously been sneaking on set of the studio to complete mocap and do voice recordings, it seems like this would be rather last-minute preparation for the movie, and we all know that Marvel Studios isn't known to play around like that.
So what does this mean for the character of Thanos? Well, it could mean a handful of things, but these are my personal predictions:
#1: Josh Brolin is solely a Voice Actor.
Adding voice to a character is something that can be done later on into development, but is usually done earlier on in production to match facial shapes best for the character to the voice.
#2: Josh Brolin will only be used for a After-Credits Scene.
Though Marvel doesn't usually film things last minute, they do often film their After-Credits scenes last minute to minimize the public's knowledge of it and guarantee everything will be a surprise for fans, viewers, and often for many of their own staff. Yeah, they keep their security high for this kind of stuff.
#3: Josh Brolin won't be in Guardians of the Galaxy.
I know, this sounds crazy at first, but hear me out. Thanos has been confirmed to have involvement with [The Avengers: Age Of Ultron](movie:293035), which just so happens to be only halfway done with it's filming production. As for Thanos' involvement with [Guardians of the Galaxy](movie:424073), all we know is that he will get more screen time than he got in The Avengers, which was maybe 3 seconds and a few voice-overs?
In my honest opinion, Josh Brolin has likely been cast as Thanos for The Avengers 2 where he will do mocap and voice, while Thanos in Guardians will likely be all CGI and possibly even voiced by the same man who did his voice in [The Avengers](movie:9040). However, it's possible that any combination of these three predictions could be true. Josh Brolin could do just the voice of Thanos during the movie, mocap & voice in an after-credits scene, and mocap and voice in Avengers 2. With Marvel, anything is possible!
Guardians of the Galaxy will hit theaters on August 1st, 2014.