Unlike the rest of the characters in the ensemble film - which stars Robert Downey Jr, Chris Hemsworth and Scarlett Johansson - it was the first time Mark appeared as the part in a Marvel film, and he reveals he immediately regretted going online to see what fans had to say about it.
He said: ''I knew what my responsibility was. I felt it just by making the mistake of going online and reading some of the fanboy responses to the announcement that I was playing the next version of Bruce Banner. That was a mistake. I will never do that again.
''But, I've never had a role be more scrutinised and criticised, even before I'd shot a single frame.''
However it was not just the fans the 'Kids Are All Right' actor was nervous about - he was also intimidated by his fellow cast-mates.
He added to Collider.com: ''Coming onto the set with all of those guys was pretty daunting. Many of my heroes in life are in this cast. So, I knew that I had big shoes to fill, so to speak. It was tough, and I wish that I had a cool costume to wear the entire time, instead of a leotard that was painted like a Chinese checkerboard.''