In an early scene of John Sayles’s recently released Honeydripper, a blues singer performs in a nearly empty ramshackle club. She’s hunched and careworn, but not without a certain regal quality. Accompanied sparely by piano and harmonica, her voice has a bottomless, meditative calm, even as she teases her man, asking him to choose her over booze. With the exception of a few low-key drunks, her audience consists of the club’s anxious owner and a nattily dressed man, who looks upon her with reverence.

This is the Honeydripper saloon–source of the movie’s title–and these are the hard times it’s facing. The aging master, Bertha Mae, has become a commercial liability. Her old-timey style is still respected, but she no longer draws a crowd the way the blaring jukebox down the road does. The club she’d headlined is saved only when a mysterious, messianic young man plugs in his guitar and brings together electricity and human presence. On a modest scale, Honeydripper dramatizes the crucial moment in black pop when the music plugged in, got more insistently rhythmic and reflected a cultural shift toward an urban environment.

Like O Brother, Where Art Thou? and Buena Vista Social Club, Honeydripper is more than a handsome film about musicians. It’s an immensely appealing total package, one with serious crossover potential. The Honeydripper All-Stars have already hit the festival circuit to rave reviews; the band includes young ax-man Gary Clark Jr., Delta revivalist Keb’ Mo’, former Howlin’ Wolf saxophonist Eddie Shaw, harpist Arthur Lee Williams and keyboardist Henderson Huggins (whose hands play those of Danny Glover’s piano-playing club owner). It also features Dr. Mable John, the versatile vocalist who nails the part of Bertha Mae. Anyone who sees the film, or catches the All-Stars live, would assume that John–like her bandmates–is a veteran of the blues scene. But onscreen, John is acting more than reliving her past. She can belt out the blues on cue, but her unscripted place in history is as a forgotten trooper of the 1960s soul era, a vocalist who cut a Zelig-like path through an epoch while leaving some minor musical triumphs behind.

John was born in 1930, in Bastrop, Louisiana. Like so many African-Americans of her generation, she eventually found her way to a big city up North–in her case, Detroit. After high school she worked for an insurance agency run by the mother of future Motown mogul Berry Gordy. Her younger brother, Little Willie John–best known for the original version of “Fever”–was a star by the mid-’50s. Little Willie’s smooth mix of exuberance and anguish is arguably the flashpoint of soul, the point at which R&B’s raw timbre discovered soaring lyrical poignancy; it’s hard to imagine Otis Redding or Marvin Gaye without Little Willie. While hardly the force of nature that her brother was, Mable could sound tough and coy in much the same way.

Although she logged the requisite hours with her church choir, John’s musical career didn’t begin in earnest until 1959, when Gordy brought her into the studio for “Who Wouldn’t Love a Man Like That.” This track, recorded for the fledgling Tamla label, made her the first female artist to record for the Motown group. Released in 1960, “Who Wouldn’t Love a Man Like That” had easy-swinging horns, hand claps and singalong appeal, as did follow-up tracks like “(I Guess There’s) No Love” and “Actions Speak Louder Than Words.” John found her way onto package shows with Jackie Wilson, her brother and Etta James. But compared with the shimmering harmonies and sculpted solos of vocal groups like the Marvelettes and the Supremes, John was just a little too robust. As she later put it, “I’m not a pop singer.” She continued to record for Gordy until 1963. But all of her tracks stayed in the vaults except a 1963 remake of “Who Wouldn’t Love a Man Like That.”

John remained affiliated with Gordy, performing live until she asked to be released in 1964. In 1965, while living in Chicago, she hooked up with a new manager, local DJ Lucky Cordell, who was connected to Memphis’s Stax Records. Unlike their rivals at Motown, the Stax brain trust believed that sharp pop instincts and a gritty, funky sensibility could happily coexist. The year John came aboard, the label released signature hits like Eddie Floyd’s “Knock on Wood,” Sam and Dave’s “Hold On! I’m Comin'” and Otis Redding’s “Try a Little Tenderness.” A 1967 Stax Revue played to sold-out crowds in Europe and made the Stax musicians realize for the first time the universal appeal of their music. Motown had better sales, but Stax had the real feeling of a movement.

Released in 1966, “Your Good Thing (Is About to End),” John’s first single for Stax, might be her finest three minutes. Her biggest hit, it broke into the top 100 on the pop charts and reached No. 6 on the R&B charts and would eventually be covered by Lou Rawls and Bonnie Raitt. Gutsy, peevish and weary, “Your Good Thing” is as unsettling as it is moving. With another singer, this minor-key Isaac Hayes/David Porter composition could sound like a dirge. But John, voice reedy atop a jumpy, out-of-tune piano and bed of moaning horns, has apprehension in her voice as she announces, “You don’t have to love me when I want it/’Cause somebody else will.” Then the rhythm section locks in, the piano vamps and the horns ooze disdain as John taunts her man with “your real good thing.” The performance nails the mix of disgust, sadness and superiority so familiar to anyone who has ever been forced to abruptly end a relationship. “Your Good Thing” was based on events in John’s life; she has said it “relieved the bitterness…like getting something off of my chest.” Available on the mammoth Complete Stax/Volt Singles (or on iTunes), it’s one of those gems that only appear minor if you aren’t paying attention, and among soul buffs it’s the crown jewel of John’s reputation. It’s also worth noting that the tune is worlds away from the voice of Bertha Mae. Bertha Mae tells a story with equal parts sass and sophistication; Mable John sounds like a character caught up in the midst of the action.

In 1967 John told her man “Don’t Hit Me No More,” a single that refuses to favor the victim or the badass. It opens with the strange lament, “I’m so sorry you had to slap me,” veering dangerously close to the Crystals’ “He Hit Me (and It Felt Like a Kiss).” Within seconds, though, John quips that this fit of jealousy at least shows that her man cares. The lyrics give us domestic violence with a context–that is, a relationship–and John’s performance is accordingly varied and confused. She’s not ordering her lover to back off or pleading for her life; her voice is alternately dismayed, pissed off, betrayed and depressed as she tries to make sense of what’s happened. The rhythm section, all chattering guitar and loose shuffle, is the soundtrack to an ordinary life that knows it doesn’t have the luxury of high drama.

It’s John’s straight style that’s cemented her status as a favorite among collectors; the power of “Don’t Hit Me No More” is the power of the familiar. But the year before, another female singer had come to Stax and recorded what may be the most devastating song in a catalog packed with them: Ruby Johnson’s “I’ll Run Your Hurt Away.” Unlike the unique “Your Good Thing,” this song follows a fairly standard Stax template. But Johnson’s plangent cries turn it into a spine-tingling plea for love conquering misery. Isaac Hayes claims to have broken down in tears while composing it, and Johnson delivers it with bottomless sympathy and strength. It’s the kind of thing I put on when all seems lost, but I can’t seriously say I’ve ever found a moment of real life this sublime.

After John left Stax in 1968–overcome, some say, with depression following her brother Willie’s death–she became the director of Ray Charles’s backup group, the Raelettes. Then, after almost ten years of life with Charles, John left the business and turned toward the light. She launched the Joy in Jesus Ministries in the Los Angeles area, as well as a homeless outreach program that’s still active. The “Dr.” before her name in the credits of Honeydripper denotes the Doctor of Divinity degree she earned in 1993. John also put her vocal talents to good use on gospel numbers, such as on the self-released album Where Can I Find Jesus? Soul singers often turn to gospel, but they don’t usually follow the third path John has taken: she has become a novelist, teaming up with music biographer David Ritz for a series about Albertina Merci, a blues singer turned evangelist. In the last two years, Random House subsidiary Harlem Moon has published Sanctified Blues and Stay Out of the Kitchen! Love Tornado follows this June, making two books in a row named after past tracks of hers. Like Sayles taking the Honeydripper All-Stars on the road, John knows what a valuable commodity credibility can be.

Pop music often tempts us with unattainable highs and lows of emotion–for a brief moment with Little Willie or Ruby Johnson, sound is bigger than life. But it’s just as moving when it reflects what we know, telling it like it is as we nod our heads in agreement. Mable John’s finest singles let listeners peek inside themselves, showing that the choir has every bit as much to say as the star.