Iron Man / Tony Stark is the heart of 'Marvel Avengers Assemble', according to Chris Evans.

The 31-year-old actor - who plays Captain America in the superhero movie - loved working with Robert Downey Jr. on the film and thinks his character is the most important one.

He explained to ''I think without Tony, we don't work. He really is the glue in the family. He is the fire, the thing that keeps you coming back. I think, at least for this movie, Cap's struggling to find his footing in this modern day - he's a fish out of water. He's a little more uncomfortable in his own skin than he normally might be, and he's not hitting the ground running without the charisma and the leadership and the character that Tony Stark is.''

While Chris sees Iron Man as instrumental to the film, Robert is equally enthusiastic about Captain America.

He said: ''I remember that one of my earliest concerns with this was, 'Haven't we done this, it's a bunch of superheroes together but they're a dysfunctional family' thing before?' But it has transcended that in a lot of ways. Obviously there is only one person whose name is Captain, and if you have a squad of people, there is a time where we need his prowess in terms of things that require actual strategic and military stuff, in what is essentially a war that we are trying to avert, of sorts. So I think that's really valuable.''