“Honeydripper,” John Sayles’s shambling fusion of pop mythology and social mosaic, imagines the world-changing moment, around 1950, in the rural South when a blues guitarist first plugged in his ax and rocked the joint.

That cultural harbinger, Sonny Blake (Gary Clark Jr.), is an itinerant, freight-train-hopping young guitar wiz with no particular place to go, who stops off at the sleepy little town of Harmony, Ala. His instrument, cut from a solid woodblock and lacking a sound hole, emits rock ’n’ roll lightning at the Honeydripper, a dilapidated roadhouse on the verge of going out of business. Saturday night in America will never be the same.

Don’t go rushing to your rock encyclopedias for confirmation that any of this is scrupulously factual. Mr. Sayles, who wrote, directed and edited “Honeydripper,” is primarily interested in fusing archetypes from the Jim Crow South, both black and white, with mythic dimensions. An affable blind bluesman, Possum (Keb’ Mo’), whose enigmatic smile hints at his possession of secret knowledge, turns up now and again. Everywhere and nowhere at once, his elusive presence helps push the movie toward the realm of fable.

Sonny, an unknown, is passed off as Guitar Sam, a regional blues legend who is a last-minute no-show at the Honeydripper. The music made for the movie by Mr. Clark, a blues hotshot from Austin, Tex., with several albums to his credit, is proto-Chuck Berry, though not as distinctive.

All the characters and situations are familiar folkloric elements in a town where flimflammery is a way of life. Desperately in debt to his landlord, the Honeydripper’s owner, Tyrone Purvis (Danny Glover), a former boogie-woogie pianist, steals the liquor destined for a rival club whose jukebox is driving him out of business.

Upon arrival in Harmony, Sonny is arbitrarily arrested for vagrancy by the white sheriff, Pugh (Stacy Keach), and dispatched to pick cotton on the plantation of Pugh’s crony, the local judge. On payday a plantation owner refuses to give a laborer his full wages simply for the pleasure of humiliating him. But the omnipresent threat of physical violence explodes only once in a black-on-black knife fight inside the Honeydripper.

While operating on a mythic level “Honeydripper” also wants to create the same kind of top-to-bottom social microcosm found in many of Mr. Sayles’s films. But this time his attempt to have his characters be simultaneously symbolic and real works at cross purposes. He is so uncomfortable writing dialogue in an old-time Southern argot that the conversations in “Honeydripper” rarely settle into the easy, colorfully idiomatic flow that has always been a hallmark of Southern speech.

Hard as they try to break through the stiffness, the film’s fine actors only fitfully succeed in camouflaging the machinery behind their characters. Two of the most familiar are a fluttery, white Southern matron, Amanda (Mary Steenburgen), who unwittingly condescends to her black maid, and an overweight black mama, Nadine (Davenia McFadden), with a dirty mind. Amanda comes straight from Tennessee Williams and Nadine from Bessie Smith.

Other one-note characters include a dignified, terminally weary blues singer, Bertha Mae (Mabel John), her devoted long-time consort Slick (Vondie Curtis Hall), Tyrone’s partner Delilah (Lisa Gay Hamilton), whose frustrations are driving her to explore Pentecostalism, and Tyrone’s best friend Maceo (Charles S. Dutton). Finally there is Tyrone’s angelic stepdaughter, China Doll (Yaya DaCosta), who becomes the inspiration for Sonny’s ultimate rock ’n’ roll blast.

“Honeydripper” is agreeable, well-intentioned and very, very slow. Sadly, it illustrates the difference between an archetype and a stereotype. When the first falls flat, it turns into the other and becomes a cliché.

The movie is rated PG-13 (Parents strongly cautioned). It has strong language and one scene of violence.

HONEYDRIPPER

Opens in New York and Los Angeles on Friday.

Written, directed and edited by John Sayles; director of photography, Dick Pope; music by Mason Daring; production designer, Toby Corbett; produced by Maggie Renzi; released by Emerging Pictures. In Manhattan at the Cinema Village, 22 East 12th Street, Greenwich Village. Running time: 2 hours 2 minutes.

WITH: Danny Glover (Tyrone Purvis), Charles S. Dutton (Maceo), Lisa Gay Hamilton (Delilah), Mary Steenburgen (Amanda Winship), Stacy Keach (Sheriff Pugh), Vondie Curtis Hall (Slick), Sean Patrick Thomas (Dex), Keb’ Mo’ (Possum), Kel Mitchell (Junebug), Gary Clark Jr. (Sonny Blake), Mabel John (Bertha Mae), Yaya DaCosta (China Doll) and Davenia McFadden (Nadine).