The actor, who voices the role of Tony Stark's (Robert Downey Jr) computer Jarvis, explained that he is never sent the full script for the films - with the exception of The Avengers, which he described as "an anomaly".
"I get brought in right at the last moment, where if they've worked out that they have a clarity issue or whatever, they can always add it to [my] dialogue," he said.
"I feel like a pirate. This is robbery. I walk in, I say some lines on a piece of paper for two hours, and then they give me a bag of money and I leave and I go about my day.
"I sort of feel guilty, because at least acting can be exhausting, with long hours… but I do nothing! And I've never seen one of them."
Bettany went on to emphasise that there are many of his movies which he has never seen, and the Marvel films just happen to be among them, saying: "It's not because of any snobbishness, it's just not my thing.
"I'm plagued by fanboys who love Jarvis... They come up and I've got no idea what they're talking about. I've got no idea about Iron Man - I don't get sent the whole script."
> Robert Downey Jr: 'Iron Man 3 could be best superhero movie ever'
Iron Man 3 is released on April 26 in the UK and May 3 in the US. A trailer will debut later this month.
Bettany's upcoming crime thriller Blood, which is directed by The Awakening helmer Nick Murphy, plays at the BFI London Film Festival this month.
Photo gallery - Marvel movies in pictures: