Late in “Welcome Home Roscoe Jenkins,” a comedy about a talk-show host (Martin Lawrence) revisiting his relatives down South, there’s a scene in which characters sit in the family patriarch’s living room playing video games. The scene is already funny because the competitors are playing on a bulky 1977 television while the hero’s recent gift to his father, a flat-screen plasma set, sits unopened in a box nearby. But the crowning touch is a throwaway detail: The TV’s knob is missing its button, and in its place is a pair of pliers clamped over the metal stem.

“Roscoe Jenkins” is just a shaggy diversion that gives its energetic star and its populous, accomplished supporting cast (including James Earl Jones as Papa Jenkins, Margaret Avery as Mamma Jenkins and Cedric the Entertainer and Mo’Nique as the hero’s siblings) a chance to clown around. But it’s a cut above other films of its type because every scene is packed with details like those pliers — touches that suggest that the film’s writer and director, Malcolm D. Lee (“The Best Man”), is working overtime to smuggle life into formula.

Mr. Lawrence is apoplectic and affecting as Roscoe, a braggart who now calls himself R. J. Stevens, and whose cockiness masks his hurt over past and present miseries.

His dad always favored Clyde (Cedric the Entertainer), an adopted child turned used-car impresario who won every prize Roscoe coveted, including Lucinda (Nicole Ari Parker, sweet but tough), Roscoe’s childhood crush and Clyde’s current squeeze. Roscoe’s gossiping, eavesdropping, trash-talking sister (Mo’Nique) stirs every pot, and when called out by Roscoe, silences him with a “Raising Arizona”-level backside whupping.

His fiancée, Bianca Kittles (a gem of a performance by Joy Bryant), is a “Survivor” prizewinner who pressures Roscoe’s sweet-natured son, Jamaal (Damani Roberts), to be more ruthless, and whose scheming nature brings out Roscoe’s least flattering qualities. (Roscoe puts up with her because the sex makes him speak in tongues.)

You can see every twist coming: Roscoe’s disenchantment with Bianca and his reconnection with his family; the tension between Clyde, who treats the empathetic Lucinda as a possession, and Roscoe, who sees her as his salvation; the sequence in which the family members run an obstacle course, and Roscoe must choose between helping his struggling son and heeding Bianca’s exhortations to abandon Jamaal and win the race. (The film’s vilification of Bianca is its least savory aspect; a story with this many screwed-up, hypercompetitive characters has no business scapegoating one of them.)

But it’s the gags that make or break this sort of film, and “Roscoe Jenkins” is aces in that department, from Clyde’s fits of comic weeping (he sounds like the Cowardly Lion) to the newly health-conscious Roscoe’s furtive sampling of forbidden barbecue (his sauce goatee gives him away) to the veteran scene-stealer Mike Epps, as Roscoe’s hustling best friend, getting a laugh simply by pronouncing the word Telemundo.

The most absurd gag is a dog sex scene, shot like something out of a soft-core film, that pays off in subsequent images of those same dogs tenderly spooning. As the song says, everybody needs somebody sometime.

“Welcome Home Roscoe Jenkins” is rated PG-13 (Parents strongly cautioned) for crude and sexual content, language and some drug references.