On any list of unlikely candidates to play the great rhythm-and-blues belter Etta James, Beyoncé Knowles would have to be up there with Sarah McLachlan, Enya and George W. Bush. And yet, here she is, in "Cadillac Records," togged out in snug '50s cocktail frocks and a bright blond wig, firing up smokes, sucking back booze straight from the bottle and flashing a mean eye at anyone uppity enough to be ogling her undercarriage as she bends over a pool table to sink a shot.

And she's convincing. She's not Etta James — there's only one of those — but Beyoncé brings a persuasive period texture to the character. We can believe this is something like the way the young Etta might have been: a woman who, in her early 20s, has already seen a lot of hard road on the R&B tour circuit and who doesn't trust success any more than she trusts anything else in life. It's a role that allows Beyoncé to demonstrate, once again, what a smart, sharp actress she is.

"Cadillac" endeavors to tell the story of Chicago's celebrated Chess Records label, where in the 1950s the primordial country blues of the Mississippi Delta got plugged in and turned way up, eventually into rock and roll. This wouldn't seem to be Beyoncé's natural musical element (although she does hail from Texas, home of such sizeable R&B stars as T-Bone Walker and Johnny "Guitar" Watson — the latter a big influence on Etta James herself). But while she's not a perfect match for James vocally, this proves to be not much of a problem, even in Etta's big performance scene in the movie, when she's recording the sublime "At Last" in the Chess studio. The song is an old big-band tune, but in 1961, backed by a white string section and carefully modulating the blues-mama power of her voice, James turned it into both an R&B smash and a top-30 pop hit — she took the scruffy Chess sound uptown. Beyoncé has none of James' hard, cutting edge; but she's a more vibrant singer — her voice practically glows at you — and she nails the song in a new, modern way. (She may also have positioned it for another run at the charts.)

That Beyoncé was also the movie's executive producer, however, may account for one of its structural problems. Any film about Chess Records has to devote considerable time to Leonard and Phil Chess, the Polish-immigrant brothers who started the label in 1950. And indeed, the picture begins with Leonard Chess (Adrian Brody) climbing up from his Southside Chicago nightclub onto a low rung in the recording business, and it ends with him selling Chess in 1969. But director Darnell Martin, who also wrote the script for the movie, has made an admirable effort to spotlight some of the many famous black performers who actually moved units for the label. So we get to spend quality time with Muddy Waters (Jeffrey Wright), the company's first big hit-maker, and with harmonica titan Little Walter (Columbus Short), who rose up out of Muddy's band to chart records on his own, and with the prolific house songwriter Willie Dixon (Cedric the Entertainer), who provides voice-over narration that may not have been entirely necessary. (Cedric seems pretty much to be playing Cedric, in any case.)

Little Walter was an unruly musical innovator (his use of over-the-top amplification brought the harmonica out of the background to stand riff-to-riff with the electric guitars), and Short plays him as the semi-tragic figure he was without shortchanging the man's ebullience. But it's the Waters character that's meant to be the heart of the picture. Wright brings an unexpected emotional warmth (and a pretty great singing voice) to his role, but he seems constrained by the dueling necessities of approximating Muddy's distinctive stone-Buddha demeanor without lapsing into blooze-man caricature. He never quite does that, but the control required to pull off this balancing act serves to make his performance a little too recessive to really hold the movie together.

Brody's Leonard Chess can't do it, either. Brody is too sweet to be persuasive as the sort of old-line label boss who might have been truly moved by the music he recorded, but was also mightily inspired by the money to be made from catering to an underserved niche market — in this case, the one for so-called "race records." The script highlights Leonard's practice of presenting his artists with a brand-new Cadillac as soon as they scored a big hit; there are passing references to the fact that the price of these cars was probably deducted from their royalties — and that Chess' accounting practices in general were casual at best — but Brody isn't given the means to illuminate this complexity. (The part might have been better served in any case by an actor with a little more weasel in his soul — Sam Rockwell, perhaps.)

These muted characterizations leave the way clear for Beyoncé to dominate much of the picture with her sheer star power. In terms of the Chess story, though, this is ridiculous. Etta James made some terrific records for the company, but its stable of artists included far more influential musicians — among them, Howlin' Wolf, Bo Diddley and Chuck Berry. Amazingly, Diddley isn't represented in the movie at all (much like Phil Chess, whose alleged appearance is so fleeting I missed it). Howlin' Wolf, though — that towering Delta Blues primitive — is portrayed with dark, riveting intensity by Eamonn Walker; and Mos Def (of all people!) dives into the character of Chuck Berry with an electrifying joy, tearing up such Berry classics as "Nadine" and "No Particular Place to Go" and duck-walking all over the place, too. He brings a perfectly weighted slyness to his few substantial scenes — especially the one in which he mock-apologizes to a group of Southern concert-bookers for not being the white guy they expected to show up for the gig. He could have run away with the whole picture — and made it a very different one — but there's just not enough of him.

"Cadillac Records" has to be applauded for the unusual care and affection with which it depicts an important time and place in American popular music. It's too bad the movie's not a little more dynamic and a little less episodic. But then in the normal course of things, it could have turned out a lot worse.

Don't miss Kurt Loder's review of "Frost/Nixon," also new in theaters this week.

Check out everything we've got on "Cadillac Records."

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