The Oscar nominee - who plays a villain-turned-hero in the video game-themed movie - suggested that the voice cast had to be in sync to create the best possible performances.
"To director Rich Moore's credit, he agreed that we should have as many sessions as we could together because you never know what's going to happen," Reilly said, reports Flicks and Bits.
He added: "Like, if I record my lines in a certain way, then I go away, then when you come in to record your lines to react off of me, it's very limited to what you can do.
"You can add a few things here and there, but for the most part you're bound by what I already did. But if we're in there together anything can happen."
Reilly went on to suggest that it was helpful to have close contact with Wreck-It Ralph's writers Jennifer Lee and Phil Johnston.
"We had the writers in the room, some great story people from Disney who were collaborating," he said. "We were forming the story in some ways - and certainly the dialogue - right there in the moment. I think that comes off on-screen seeming very alive and happening in the moment."
Wreck-It Ralph also features the voices of Sarah Silverman, Jane Lynch and 30 Rock's Jack McBrayer.
The movie opens on November 2 in the US and February 15, 2013 in the UK.