Michael Mann should be honored for his talent at sleight-of-hand. In 1999’s “The Insider,” the writer/director took a dry subject (ethics in journalism) and made it riveting. In his new film, “Ali,” he takes an interesting topic (Muhammad Ali’s career) and makes it dull. Presto!

This 157-minute film begins and ends with Ali in a boxing match, first against Sonny Liston in 1964, and then against George Foreman in 1974. Both of these matches are shown approximately in real time, somewhat compressed from how long they actually took, but still without any cutting away.

The movie in between has the same real-time feel. Some scenes, already about nothing, go on interminably. We see a lot of things without ever getting an indication of why Mann saw fit to show them to us.

Curiously, there are some important details Mann leaves out. For example, the name of Ali’s second wife. Or what city it all takes place in. (Maybe you already know Ali was from Louisville; maybe you don’t.) Or what year it is: After the caption says 1964 in the first scene, we are never told again. Suddenly he’s getting divorced, and remarried, and there’s a child, all with no sense that any time has passed. The only method the film gives you to know the final match is in 1974 is to have someone say Ali is not the same fighter he was “10 years ago,” and even that is obviously open to a lot of different interpretations.

Will Smith plays Cassius Clay, who changes his name to Muhammad Ali as the result of his affiliation with the Black Muslims. A big chunk of the movie focuses on his attempt to avoid being drafted to Vietnam due to conscientious objection — but then, when he wins his court appeal and doesn’t go to jail, it’s glossed over quickly.

That sense of detachment pervades the movie. We see a lot of events, and hear a lot of talking, but it never seems to mean anything in terms of who these people are. The most interesting relationship is between Ali and Howard Cosell (an unrecognizable Jon Voight), but even that is more entertaining than insightful. (“You want some food for that thing?” Ali says in regards to Cosell’s toupee.)

Smith nails Ali’s swagger and the cadences of his speech; like Jim Carrey as Andy Kaufman a couple years ago, he’s not playing the role so much as channeling it. There’s a fine line between doing an impersonation and playing a character. “Ali” points out that sometimes the only difference is that impersonations allow you to wear wigs and makeup (see Jon Voight), while characters require you to bulk up your muscles and change your hairstyle. But in neither case are we actually seeing a real person; it’s still just a caricature.

There’s a parade of supporting characters: Jamie Foxx as Ali’s Jewish friend Bundini, Mario Van Peebles as Malcolm X, Mykelti Williamson as Don King, Giancarlo Esposito as Ali’s father. What was it about Ali that attracted them to him? Who was Ali, and why did he do what he did? Why was America so fascinated with him? Someday, maybe a filmmaker will spend 157 minutes answering those questions.

C (2 hrs., 37 min.; R, some harsh profanity, some brief sexuality, a lot of boxing violence and some blood.)