Remember the fake nose that Matt Damon wore in “Ocean’s Thirteen”? The 30 extra pounds he sports in Steven Soderbergh’s latest movie, “The Informant!” provide a fuller transformation, albeit a less flattering one. The movie itself is also more substantial than “Ocean’s Thirteen,” but despite a promising setup and a solid cast, the humor in “The Informant!” wears thin before long.

The film is based on the true story of Mark Whitacre (Matt Damon), an executive at the Archer Daniels-Midland agricultural company who worked as an informant for the FBI in the early 90s. At the movie’s start, Whitacre seems to be a simple biotechnology worker appalled at the corruption in the company’s business practices. It is out of the goodness of his heart, or so he claims, that he volunteers to inform on ADM’s price fixing agreements.

As a spy, Mark Whitacre is nothing like Jason Bourne. Matt Damon is almost unrecognizable beneath his round face, thick glasses, toupee, and moustache. He easily charms us with his stutter, awkwardness, and apparent good nature, even as he utters one bold-faced lie after another. Damon adeptly tricks us into trusting his character, despite mounting evidence that we should not.

The audience is privy to nearly all of Whitacre’s thoughts as he bumbles through his double life. His stream-of-consciousness voiceovers begin as the most amusing moments of the film. Even while holding conversations with his colleagues at ADM and the FBI, Whitacre contemplates tie patterns, butterfly adaptations, and the hunting techniques of polar bears. His nonsensical musings quickly reveal his atypical understanding of everyday interactions. During his first one-on-one meeting with his FBI handler, we hear him contemplate a potential friendship with the agent: “I could see us fishing, or whatever.”

With a keen attention to detail, Soderbergh sets a fittingly rich background for Whitacre’s frenetic imagination. His frequent close ups-of black and green screen computers and clunky recording devices serve as fond reminders of early 90s technology. Even Whitacre’s paranoid fantasies are very much a product of his decade; at one point he compares his own situation to that of Tom Cruise in “The Firm,” which opened in 1993. In nearly every scene, the screen is bathed in brown and yellow tints, adding to the warm, nostalgic feel of the movie.

Unfortunately, the comedy of Whitacre’s free association wears off over the course of the film. As it becomes apparent that Whitacre is leading more than one kind of double life, the movie’s unvarying cheerful tone begins to feel increasingly grating. The same upbeat, repetitive music that accompanies Whitacre’s first jaunty walk into his office also plays when the FBI conducts a raid on ADM.

This overwhelming cheer stems from the narration, which is almost entirely from Whitacre’s delusional point of view. The narration also renders the supporting characters rather flat. From the moment her husband gets involved with the FBI, Whitacre’s anxious wife (Melanie Lynskey) urges him to come clean: “Whatever you do, Corky, no matter what’s going on, just be honest with them and tell the truth.” But the audience forgets her presence in the film almost as easily as Whitacre forgets her advice. His wheedling coworkers suffer a similar fate; it’s only really possible to differentiate the other executives at ADM by their horrible ties. Most tragically, the comedic gifts of Toby Hale—known best for his role as Buster Bluth in “Arrested Development”—are squandered during his brief appearance as Whitacre’s lawyer.

Near the end of the movie, as Whitacre’s plans are beginning to unravel, he notices a swarm of ants feasting on discarded food crumbs. “That’s a big break right there,” he thinks to himself, seemingly envious of their unique position. “There are no choices to be made,” he continues. “You’re an ant, you just eat it.” Whitacre’s own motivations don’t seem so different. Every time he sees an opportunity for self-gain, he takes it without hesitation. Needless to say, such an approach to life works less well for humans than it does for ants. As Whitacre’s life spins towards disaster, his false cheer and bizarre jokes don’t stop, but as we watch him stutter, bluster, and lie his way through meeting after meeting, it’s difficult to keep laughing.

—Staff writer Rachel A. Burns can be reached at rburns@fas.harvard.edu.