The 47-year-old Oscar nominee (for 2002's "Chicago") initially didn't want the job, reticent about the scope of the project and the process of making animated movies.
"I'd heard that making animated movies for actors was often kind of this isolated, strange experience," Reilly said. "You go in and do your lines, and then you react to recorded lines from other people, and you're never in the same room at the same time and maybe you meet them at the premiere, the other actors."
He'd been told that animated films were made by committee, rather than the director's vision guiding the project. And the previous animated roles he was offered never provided a complete script.
"I was like, that doesn't sound fun at all," he said. "That just sounds kind of like just an empty suit kind of job."
But that wasn't his experience on "Wreck-It Ralph." Reilly attended story meetings, met with animators to develop the look of his character and worked side by side with co-stars Jack McBrayer, Jane Lynch and Sarah Silverman.
"We improvised a ton," he said. "I really, really loved it."
He did motion studies with the animation team, which incorporated Reilly's gestures and movements into the look of the massive-handed, 9-foot-tall, 643-pound Ralph.
"I was like, `This is how he gets up from a chair. It's like an eight-step process. It's kind of like an elephant raising itself off the ground,'" the actor recalled, adding that he recognized himself in the character, especially in "the secondary gestures, like picking -at-himself kind of things."
"Every single gesture and expression that a character makes in an animated movie means hundreds of hours, if not thousands of hours of work from a bunch of people," Reilly said, "so I was really happy that they made the commitment to do those little subtle things to really finish out the character in a really detailed way like that."
The filmmakers appreciated Reilly's contributions so much that the actor earned a story credit.
"It turned into this dream job," the actor said. "I'm really glad that I took my time trying to find the right animated thing to do first because this turned into something I can be really proud of, and that I put a lot of heart into, you know?
"If you're going to have an avatar or something like that follow you for the rest of your life, it might as well be something that you really feel like you put a lot of heart into. That it's not just a toy. It's a project that you really believe in."
(Copyright 2012 by The Associated Press. All Rights Reserved.)