April 21, 2017 at 01:30 PM EDT

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Guardians of the Galaxy Vol. 2 isn’t out until next month, but many of the superhero sequel’s core cast, including the Peter Quill-playing Christ Pratt, have already shot scenes for Avengers: Infinity War, which is being directed by the Russo brothers and will be released May 4, 2018. So, what has that been like?

“It was really incredible,” says Pratt. “It seemed like a dream, as so much of these past few years has felt, like a weird fever dream where you’re waking up and you’re not sure if you’re hallucinating or not.”

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Pratt says that working with Robert Downey Jr. was a treat and an exciting challenge, as well as the culmination of a long-in-the-works plan by Guardians franchise director James Gunn.

“Early on, James said that, when he was casting the role of Quill, he was thinking of somebody who was gonna one day be in a scene opposite Robert Downey Jr. He knew that character had that potential, that there was a collision course for these characters, potentially,” he explains. “At that time, that was really daunting to think of that. And over the past couple of years I’ve been slowly learning, and honing my job and my skill set for this job.”

“Getting to work with Robert, it just felt really really natural. He’s a remarkable actor,” Pratt adds. “I’d heard stories abut his work ethic, and his specific style of acting in regard to being in the moment, changing things up, keeping you on your toes. Essentially, it’s like playing with a great jazz musician or something like that. You kind of work on your chops for years, thinking there’s a chance you’re going to get a chance to work with this great jazz musician, and then you do, and you watch him go off on some solo, and you’re pumped because you know that after four bars you’re going to get a chance to come in and do your own thing, and you sit there and you make some good music. It was really cool. I felt like I was playing in part of a supergroup or something.”

Guardians of the Galaxy Vol. 2 hits theaters on May 5.

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