But Robert Downey Jr has ambitions to play another role - that of an investor in technology to combat climate change.
He tells Tech Tent he wants to step out of the make-believe world of his Avengers character and confront the real threat to the planet.
"I'm not a kid any more. And I really want to find things that I think are dynamic and interesting and - tell me if I'm wrong - but isn't this the space to be catching the buzz right now?"
Downey is leading the Footprint Coalition, an investment fund that will try to spot fast-growing green technology businesses. He says much of his role will be about using his skills to tell a story about climate change, and the technology that can beat it, to his 100 million social media fans.
"I realised a few years ago that I'd amassed a pretty wide audience, I guess you call them followers nowadays. And it seemed like just posting more videos of me dancing with my alpaca wasn't really moving the conversation on this existential threat."
Speaking over a video link from his home in California, he says a key moment in his awakening to this threat came in the wildfires in Malibu in 2018.
"I had my hoses out trying to help my neighbour not lose his house. You kind of go - this is not the rhythm that was going on in California in the 80s or the 90s."
The Footprint Coalition fund has already invested in a number of green businesses, including Cloud Paper - which makes toilet paper out of bamboo and has a mission to save a billion trees - and Ynsect, which turns mealworms into fertiliser and other products.
The Hollywood star, who played Iron Man Tony Stark in 10 Marvel films, says he had originally thought about focusing on AI and robotics, like his fictional counterpart: "I figured that was my strong suit, it was probably easiest to identify me with my namesake."
But he says he is now looking at areas like transport, energy and consumer products.
There are some real challenges for this mission.
Green technology investment is a crowded field and plenty of ideas that look attractive for a while end up struggling to make a financial return. Then there is the debate over climate change in the United States, with many politicians and voters not convinced the threat is real.
He acknowledges this culture war, remembering that when the original Iron Man movie came out "all of my conservative friends said 'you really stuck it to those lefties' and all my liberal friends said 'you really showed them'".
But he insists he can find a way to reach across the divide.
"I think there's a way to appeal to my brothers and sisters in the heartland and my conservative friends, " he says, "rather than saying, you're wrong, you're crazy and you're bad."
I tried to get Downey to talk about Elon Musk, seen by many of his admirers as the nearest thing to a real-world Tony Stark. But he was cautious.
"We have a very interesting history. I respect Elon a lot. But I have not really spoken to him in some while," he said.
But then he started talking about the importance of putting principles before personality, and of team spirit, "rather than just thinking I will say and do as I feel". The message seemed to be that Musk needed to be just a little more reflective before speaking out.
What then, does Downey hope he can achieve with his fund?
Well, first and foremost he's clear about the commercial imperative: "I want people to have gotten a well above average return."
But he also hopes that some of the businesses he backs will produce technology that has a real impact on the world - "and I would feel like I had some small part in that."