Comedian TJ Miller has a very distinct voice, which is why he's a popular guy when it comes to doing voice work. He voices Tuffnut in both the How to Train Your Dragon movies and TV series, and lends his voice to Disney's Gravity Falls. It may be that last gig that caught the studio's attention because Miller has revealed he has a voice role in Disney/Marvel's Big Hero 6.

Miller: "I’m in Disney’s “Big Hero 6,” which will be their big release this Thanksgiving. It ranges, usually it’s the stoner-slacker, or you know, a guy who is very immature. Immature but confident."
In the comics, Fredzilla can do exactly what you think he can do, namely transform into a Godzilla-like creature. They may be tweaking his character for the film, though, as he's described as a comic book geek, lab rat, and aspiring filmmaker who wears a monster suit.
Disney plans on releasing Big Hero 6 on November 7th 2014.