Even in Hollywood, there’s such a thing as artistic hubris. And no, Vin Diesel’s believing he could pull his weight in Sidney Lumet’s latest courtroom drama isn’t this week’s best example. It would be Sidney Lumet’s believing it. The veteran of 12 Angry Men and The Verdict may have a reputation for being an actor’s director, capable of coaxing a great performance from just about anybody, but Find Me Guilty suggests that Lumet might have finally reached the limits of his talent. Vin Diesel? Didn’t anyone ever tell him the one about putting lipstick on a pig?

To Lumet’s credit, Diesel does manage to just barely elude slaughter. With hair on his usually smooth pate and some significant tubbiness around his usually taut waistline, he plays Giacomo “Jackie Dee” DiNorscio, a real-life mobster who was involved in the longest Mafia trial in U.S. history—21 months—in the late ’80s. Jackie was already serving 30 years for a drug bust when the case, which involved 19 other defendants charged under the 1970 RICO Act, got under way. Of course, Jackie was offered a deal if he’d testify against the others. Of course, he turned it down. And then, of course, the self-proclaimed “gagster, not gangster” with a sixth-grade education decided to become his own lawyer.

Lumet co-wrote the court-transcript-based script with freshman screenwriters T.J. Mancini and Robert McCrea. If the director meant to make Find Me Guilty feel as drawn out and claustrophobic as the mind-boggling real trial, he most certainly succeeded. Besides the opening scenes in which Jackie gets shot and arrested, nearly all of the film’s 125 minutes take place either in Jackie’s cell or in the stuffed-to-capacity courtroom. The days of the trial are ticked off as Sean Kierney (Linus Roache), the never-lost-a-case prosecutor, trots out his seemingly endless parade of witnesses and evidence before a patient judge (Ron Silver). Lumet’s tight shots during cross-examinations only increase the sense of closeness, and when the film moves outside in its final chapters, the change of venue is like a breath of fresh air.

Inside or out, though, the focus is on Jackie, who’s portrayed not only as a thug with a heart of gold, but as quite the jokester, as well. (When the judge, trying to dissuade Jackie from defending himself, asks him if he’s had any legal experience, Jackie replies, “I’ve been in prison half my life. Sometimes I think I had too much legal experience!”) Diesel, who’s been laughable in even his typical tough-guy roles, is simply doofy tossing off one-liners here.

Jackie is meant to be the kind of charmer who can turn a trial into a comedy act as those in the courtroom bust their guts with every joke. But combined with his character’s bad suits and worse hair, Diesel’s gravelly voice, slow delivery, and ear-to-ear smiles make Jackie seem more like a corny uncle than an ingratiating smooth-talker. (When told that a “lady juror” was overheard saying she thought Jackie was cute, Kierney understandably cries out, “What the fuck is wrong with these people?”) Even when a punch line does hit—and, to be fair, quite a few of them do—it’s hard not to imagine how much funnier it would be coming from someone else.

And if Jackie fails to win you over, Find Me Guilty makes for a long sit. We meet—or rather, don’t meet—his cronies only after they’ve been herded into the courtroom, which makes it difficult to care about the trial’s outcome. There are, however, superb supporting performances to help pass the time, notably by Peter Dinklage as a skilled defense attorney, by Annabella Sciorra as Jackie’s ex-wife, and by Alex Rocco as the family’s don. In one of the movie’s best scenes, Rocco’s kingpin needs only a few words to fill a giant, crowded lunchroom with gut-wrenching tension.

Mercifully, Diesel isn’t made to play the clown the entire time, and he does make a few of Jackie’s cross-examinations fiery and compelling. But these scenes only underscore Lumet’s unsuccessful attempt to balance farce with, well, what must be satire. He uses an extended version of Louis Prima’s “When You’re Smiling (The Whole World Smiles With You)” here and there to lend a lighthearted tone, yet these are a bunch of murderers we’re supposed to be cheering on. If Lumet is trying to critique our collective cultural love of the goombah-loyal Mafia soldier, the message never really gets sent. Like Diesel’s makeover, Find Me Guilty is purely superficial.