With 'The Avengers: Age of Ultron' set for release in parts of the world this week, Robert Downey Jr. took some time to talk about his role as Tony Stark/Iron Man - the character who started the whole Marvel Cinematic Universe back in 2008. But, as it turns out, when the cast got together in 2012 to film the first 'Avengers' movie, none of them really knew just how big it would turn out to be.

Robert Downey Jr. in 'The Avengers: Age of Ultron'Robert Downey Jr. in 'The Avengers: Age of Ultron'

"I don't think any of us really expected how huge the first one would be, so it can almost intimidate you a bit because you're like, 'Oh, well we were just kinda hoping it would work out', and now there's this expectation to deliver". Presumably they weren't expecting the film to become the third highest grossing movie of all time (behind 'Avatar' and 'Titanic') with 1,518,594,910 USD worldwide in 2012.

More: The Vision Actor Paul Bettany Claims 'The Avengers: Age of Ultron' Is "Biggest Thing Marvel Has Ever Done"

"You always hope that it just defies expectation," explained Downey Jr., on the subject of sequels, "and then I guess this time around we just really wanted people not to say, 'Well, that's alright; the first one was great so we saw it anyway'." Downey Jr. explained that he hopes the inclusion of so many new cast members will bring a lot of freshness to the film.

When asked about his relationship with director Joss Whedone, he explained that "It's a respect, you know? And every director/actor relationship is so different and Joss is very in control and really wants to have authorship over things, so I think for me it was just about being a little bit more receptive". Don't think that that ownership is a bad thing, though, as according to Downey Jr.,  "In some ways, it makes the job even easier because he's just got it all figured out".

More: 'The Avengers' Stars Hit The Red Carpet For 'Age Of Ultron's' LA Premiere

"But the nice thing is, it really is a great cast of folks, and we genuinely enjoy making this entertainment together," he added. 'The Avengers: Age of Ultron' is released theatrically in the US on 1st May 2015.

Watch the trailer for The Avengers here: