Show, don’t tell is one of the oldest rules of writing. And “Something From Nothing,” the iconic gangsta rapper Ice-T’s ambitious hip-hop documentary, has too much telling and not enough showing. Three dozen mostly old-school artists weigh in, and too many of them just don’t say enough to deserve as much screen time as they get.

Boil this film down to the highlights, however, and it has plenty to recommend it. The format is bare bones, mostly interviews and off-the-cuff rhymes framed by shots of urban scenes in New York and Los Angeles (rendered like sepia-toned postcards). “Something From Nothing” eschews the celebrity aspect of hip-hop stardom to focus on the craft of the form, how its practitioners write lyrics and rhymes to create what the film’s subtitle calls “The Art of Rap.”

If you think there’s nothing more to rapping than throwing out random rhymes over a recorded loop, “Something From Nothing” will disabuse you of that notion right quick. The eternal struggle of getting the right words onto paper is universal.

“Had the [expletive] right there,” Grandmaster Caz mutters at one point. Anybody who has ever written anything will be able to relate.

There are some puzzling omissions, especially Kurtis Blow and Jay-Z, but “Something From Nothing” has plenty of charisma on the screen. Some of the film’s more entertaining bits include Rakim attempting to explain his 16-dots-on-a-paper method of lyric writing (which even Ice-T admits he doesn’t understand); Immortal Technique explaining why he writes while in a state of physical hunger; Wu-Tang Clan’s Raekwon holding forth on the nunances of the word “wack”; and KRS-One recalling the start of his hip-hop career – getting called out for his clothes at someone else’s hip-hop battle. Ice-T has enough credibility and history to command respect from his peers, even when confessing that he’ll sometimes fake onstage microphone problems to cover when he forgets lyrics.

Eminem makes the most vivid impression. Spitting out the rhymes to the “8 Mile” theme “Lose Yourself,” he appears wild-eyed and on the verge of collapsing into a trance. You get a sense of him as a deeply damaged human who copes by working nonstop.

At the other end of the spectrum is Snoop Dogg, who treats rap like sport – a series of challenges to rise up to. Mellow and cool behind his shades, Snoop looks like he’s turning into John Lee Hooker, a wizened old bluesman. Or maybe a guy hanging around a barbershop spinning rhymes in an endless dozens game, which is how rap began in the first place.

And so the circle continues.

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