First, we saw him in the post-credits scene in The Avengers. Then, Josh Brolin (who will play him) showed up at the Avengers: Age of Ultron Comic-Con panel. He is omnipresent in Guardians of the Galaxy.
However, despite all these signs to the contrary, we will not see legendary bad guy Thanos in the upcoming Avengers: Age of Ultron.
At a press event for Guardians of the Galaxy, Kevin Feige (president of Marvel Studios) had the following to say about including the series’ biggest villain in the next Avengers movie, which is currently filming:
He’s not a part of Avengers 2. I think Thanos does what he wants, and shows up where he wants to, and I’m not going to tell him otherwise. You don’t know exactly; smirking at the end of Avengers. By the way, we’re still making Avengers 2, so nothing’s definitive one way or the other, actually.
That is not to say we won’t see Thanos at all, but it looks like it will be another cameo (at best) for the Infinity Gauntlet-seeking supervillain. The plan, it seems, has always been to make him the villain for the third movie in the trilogy, when the Guardians of the Galaxy and Avengers team up for a final showdown.
For those hoping to see those bright blue eyes next summer when Age of Ultron hits theaters, we’re sorry to disappoint you. You’ll just have to settle for Chris Hemsworth.