The Parks and Recreation star said that he signed a deal to play Star-Lord in the Marvel Studios film before he was even allowed to see the screenplay.
"Just the fact that he was part of the Marvel brand was really exciting," Pratt told Fandango.
"I was signed up to do the movie before I was even allowed to read a script, so it wasn't like something about this particular character that got me to do it. The fact that it was a Marvel movie is what got me to do it.
"Granted, once I did read it there were so many things about the character that I love, and one thing is that he's very much a kid at heart. He's like a man-child.
"And I like the idea that he's got a false sense of bravado. Deep down inside he's lonely and desperate. But on the outside he walks around like he's big and tough, and I don't think he does a great job of convincing everybody that he's not just a scared little child."
Zoe Saldana, Dave Bautista, Lee Pace, Karen Gillan, Benicio Del Toro, John C Reilly and Glenn Close will star in the James Gunn-directed film.
Guardians of the Galaxy will be released on August 1, 2014.
'Guardians of the Galaxy' filming - photo gallery: