Watching Kasi Lemmons’ biographical film about Washington, D.C. radio and TV personality Ralph Waldo “Petey” Greene, Jr. is rather like sitting through two distinct mini-movies. The first, taking the story to April 4, 1968, is a raucously funny comedy. The second, continuing the narrative from the assassination of Martin Luther King, Jr., still has moments of humor, but they’re much darker, the overall mood bleaker and more dramatic. The good news is that though there’s tension between the two halves of “Talk to Me,” both of them are quite good. The better news is that both showcase strong performances, especially from Don Cheadle as Greene and Chiwetel Ejiofor as Dewey Hughes, the program director who puts the unlikely guy on the air, but from an array of colorful supporting players, too.

Greene is a cocky convict doing broadcasts for his fellow inmates at Virginia’s Lorton prison when he first encounters Hughes, the straightlaced yuppie executive from WOL-AM, who’s reluctantly visiting his very different, incarcerated brother Milo (Mike Epps). After winning early release, Greene sweeps into WOL with his flamboyant girlfriend Vernell (Taraji P. Henson) on his arm to demand a DJ job that he claims Hughes had promised him. Despite the misgivings of owner E.G. Sonderling (Martin Sheen), Hughes gives the insistent Greene (who even gets a picket line going outside the offices) an audition, and after a stumbling start his provocative, streetwise riffs make him a local media star, overshadowing station stalwarts “Nighthawk” Bob Terry (Cedric the Entertainer) and “Sunny Jim” Kelsey (Vondie Curtis Hall). The ebullient, abrasive Greene enjoys the new celebrity, though his antics do cause trouble with the possessive Vernell.

The wild, colorful mood of the picture’s first hour changes to something decidedly more serious when Greene shows unexpected depth and maturity in the aftermath of the King assassination and the riots that followed. Seeing more to the DJ than just radio success, Hughes becomes his manager, arranging a local TV series showcasing his outrageous take on things and, eventually, a shot on the program Hughes has always idolized—Johnny Carson’s “Tonight Show.” But Greene’s aversion to moving into the mainstream, as well as the toll his lifestyle is taking on him, prove insurmountable obstacles to the sort of success Hughes is lusting after. The picture ends with a years-later coda that brings the two men back together after a long separation, and a finale that’s part celebration and part mourning.

That closing mixture points up the central weakness of the movie, which never quite manages to satisfactorily juggle the breezy, rambunctious goofiness of the first half with the grimmer tone of the second, in which it also becomes more obvious that the script by Michael Genet and Rick Famuyiwa is hewing pretty closely to the rather hoary conventions of “behind the music” (or, in this case, microphone) biography. (The sprightlier first part actuall has greater impact, which is a real structural problem.)

But “Talk to Me” wins you over, despite the screenplay’s weaknesses (especially in the second half), on the strength of Lemmons’ obvious commitment and the performances she gets from a fine cast. Cheadle contributes another amazing turn, capturing the profanely funny side of the character without sacrificing the deeper one that comes out in the film’s later stages; he succeeds in presenting Greene as a complex figure, boldly self-promoting but also damaged inside. His exuberance is matched, if not exceeded, by Henson, whose high-pitched, hard-bitten but oddly vulnerable Vernell proves a great comic foil. One might expect that anybody playing straight man to these two would emerge outclassed, but that’s not the case with Ejiofor, who holds his own even against such formidable exhibitions. He succeeds in making Hughes a complicated person as well, though one simmering at lower wattage than Greene. The three leads are aided by sharp support from Cedric, Hall, and Sheen in parts that might have been thankless but are vivid in their hands.

“Talk to Me” was obviously made on a tight budget (a fact most apparent in the crowd scenes at mid-film), but it’s technically accomplished. Stephane Fontaine’s widescreen cinematography nicely showcases the good period feel caught by Warren Alan Young’s production design, Patrick Banister’s art direction, the set decoration by Cal Loucks and Patricia Cuccia and Gersha Phillips’ costumes. An equally important contribution to the ambience is the choice collection of pop hits on the soundtrack, courtesy of Barry Cole, and Terrence Blanchard’s evocative background score.

“Talk to Me” obviously deals with a specific time and place and particular people. But in the end it speaks to us all, and for the most part does so quite effectively.