He was "born with a glitch" that causes him to feel "multiple expressions".
Miller's casting was announced as a part of World Emoji Day, with the studio releasing a fake audition reel on Twitter.
T.J. Miller will lend his voice to "EmojiMovie: Express Yourself" in a leading role, Sony Animation revealed on Sunday, July 17.
The 35-year-old comedian-and-actor will provide the voice for lead character Gene, who was supposed to be a "meh" emoji like his parents in the story.
The 57-second joke video shows Miller imitating a series of popular emojis (Kissy face, Fried shrimp), apparently under the impression that he has been picked for a live-action emoji feature - before being corrected and told that it will actually be animated.
Miller knows from tech-themed and alt-universe projects: He plays Pied Piper investor and dubious incubator Erlich Bachman on HBO comedy Silicon Valley, and he's now filming Steven Spielberg's adaptation of Ready Player One, playing the part of online troll i-R0k.
The film is directed by Anthony Leondis from script he co-wrote with Eric Siegel.
Produced by Sony Pictures Animation, "Emoji Movie: Express Yourself" will be released on August 11, 2017 by Columbia Pictures.