The 47-year-old actor, who plays Ant-Man, said he couldn't help going full nerd over his fellow Marvel superheroes during the shooting, reported People magazine.
"Being the fanboy in the group, there was very little acting required. They've all worked together and done this before - I've just seen all the Marvel movies.
"So to be there on the day (the other heroes arrived), I kind of couldn't stop geeking out about it: 'Oh my God, there's the Shield!' When I was getting the suit on..It was like, 'Oh, there's Iron Man's suit!' I did feel that, that excitement of, 'What? I can't believe that I landed here. This is nuts.'," Rudd said.
Robert Downey Jr returns to the franchise as billionaire Tony Stark and armoured alter ego Iron Man.
Also starring Scarlett Johanssen, Anthony Mackie, Elizabeth Olsen and Jeremy Renner, the film will hit the theatres on May 6.