Chris Rock was right at the Oscars: Having Russell Crowe in a movie makes a huge difference.

“Cinderella Man” is flawed, but one of its scenes will show up in movie montages forever, just like Jimmy Stewart shouting on the Senate floor in “Mr. Smith Goes to Washington” and Dorothy saying goodbye in “The Wizard of Oz.” It features Crowe as Jim Braddock, a wealthy boxer who lost it all in the Depression. Now, his family living in what appears to be a large milk crate, he has gone to a meeting of boxing executives to beg for enough money to keep the heat on.

It is a moving, complicated scene. The wealthy executives’ reactions to their former colleague range from compassion to horror to scorn. Braddock is humiliated and desperate but also brave – he will do anything to save his wife and young children. And Crowe, using little more than his eyes (there’s scant dialogue in the scene and no music), summons so much pain, hope and heart that you feel it all with him. Crowe, looking like he hasn’t slept since “Gladiator,” has no light touch, but when a scene calls for gravity, there is no one better.

The first half of “Cinderella Man” is just as good, a demonstration of Ron Howard’s growth as a director. Tiny details accrue, giving us a sense of the issues Braddock and his wife (Renee Zellweger) face as Braddock, who was banned from boxing after a few injuries, fights for another chance at the only way he knows to earn a living. For instance, Howard, Crowe and Zellweger craft a beautiful moment in which the Braddocks, at the end of their ropes, nevertheless find a way to assure each other they can make it.

Russell Crowe plays boxer Jim Braddock and Renee Zellweger portrays his wife, Mae, in the film Cinderella

But did that scene have to be witnessed by a cherubic child whom the camera lingers over? Howard has become a subtler filmmaker, demonstrated by his handling of the idea that boxing is a sport in which wealthy people pay to see poor people beat each other senseless. If Howard had made a movie about class and the Depression earlier, it would undoubtedly have featured a scene in which a fat, rich kid sucked a lollipop as he skipped by starving orphans on a breadline. But here, he makes his points about American haves and have-nots in quieter ways.

Still, Howard often acts like we need spoon-feeding. At one point, titles tell us it’s 1933, “Four Years Into the Great Depression,” as if we wouldn’t know that (why doesn’t he also tell us New York City is “A City in New York?”). And, although Howard has already established that the only reason Braddock fights is for his family, when Braddock finally gets in the ring for his comeback, Howard can’t resist having Braddock picture his apple-cheeked children and the empty cupboards in their kitchen. Why doesn’t the movie care about his opponents’ motives? Didn’t they have hungry families, too?

For a variety of reasons, “Cinderella Man” loses its way in the second half. Zellweger, who’s adept at playing the meat of a scene but can’t go deeper to find unexpected, conflicting emotions, begins to get on our nerves. Howard succumbs more frequently to sentiment. And the climax, a heavyweight championship bout against brutal Max Baer (Craig Bierko, utterly transformed), is not as thrilling or suspenseful as it should be.

After a muscular, spectacular first hour, the last hour of “Cinderella Man” becomes tentative and wimpy.