“I am not the guy who has to worry about him being replaced. He is Iron Man at this point. When I heard that he was cast I knew that something had just changed, and I usually don’t know stuff like that. I was like ‘Now we are in a new phase.’ The way he embodies it… I love working with him. He is so perfect for it, Shane captures it so perfectly in this movie.
“I want him to play it forever. The great thing is that he has a flying metal suit. So it’s not like those Charles Bronson or Roger Moore movies where you’re like ‘Dude, you can barely stand.’ He’s got some fight in him. At the same time, if Robert decides to hang it up, he has a perfectly good excuse.”
When asked why The Avengers should worry about Thanos, Whedon teased this:
“Well, Thanos is more powerful. He is so powerful, he is not someone you can just trot out and punch him. Like he did in the comics, you want him to be threading through the universe and to save the big finale for the big finale. He is definitely a part of what I’ve got going on. The thing about the Avengers is that they are very powerful, but they’re not very stable. So, there will definitely be some people that will shake them up in the next installment.”
The site suggests that this could mean Thanos won’t be defeated until a third “Avengers” film, but this is just speculation. Check out the full interview below.