The actor provides voice for Thanos whose first appearance was briefly included in the end of 2012's film 'The Avengers'.
Josh Brolin is secretly part of "Guardians of the Galaxy" cast. The actor reportedly was cast months ago as the voice of the Marvel movie's villain Thanos and had completed his work.
Thanos was first previewed at the end of 2012's "The Avengers", being played by Damion Poitier. The baddie's appearance in the upcoming "Galaxy" will help connect the film with "The Avengers" sequel, "Avengers: Age of Ultron", which is slated to hit theaters May 1, 2015.
The purple-skinned villain is a mutant member of the race of superhumans known as the Titanian Eternals. He can absorb and project vast quantities of cosmic energy and is capable of telekinesis, telepathy and matter manipulation.
Thanos' appearance in "Age of Ultron" is yet confirmed but "Guardians of the Galaxy" director James Gunn has hinted at a link between the two franchises that would make it possible. ''We're definitely connected to Avengers 3,'' he told Entertainment Weekly last month.
"Guardians of the Galaxy" hits theaters August 1 with Lee Pace playing the main villain Ronan the Accuser. Meanwhile, James Spader will play the lead villain Ultron in the "Avengers" sequel.