Robert Downey Jr. feels like a ''Big Brother'' to Scarlett Johansson.

The 46-year-old actor - who will star alongside the blonde beauty in 'Avengers Assemble' - has grown attached to his cast mates while shooting the superhero film, and he sees Scarlett, who plays the Black Widow, as a sister figure having previously worked with her on 'Iron Man 2'.

When asked what his opinions of his fellow Avengers are, he replied: ''I feel like a big brother to Scarlett and Chris Hemsworth has the whole package.

''Jeremy Renner is a serious actor who doesn't take himself seriously at all.''

Robert is reprising his role of Tony Stark and his Alter Ego Iron Man - who he played in the 2008 movie of the same name and its 2010 sequel - in the forthcoming film, which is written and directed by Joss Whedon, but he had concerns about doing it without his 'Iron Man' helmer Jon Favreau.

He added: ''He's like a brother. You don't know whether you can't live without him or you want to strangle him in his sleep. I just love the guy.''

But the 'Tropic Thunder' star admits Joss was a ''perfect'' writer and director for the movie.

He explained to Total Film magazine: ''He was perfect. During a scene I could say, 'Is this right?' Ten minutes later he had three pages of options for me to try.''