Adam Del Deo and James D. Stern’s film is a simple showbiz-elucidation machine, built from the most basic and durable parts. First: the tape-recorded late-night bull session with a studio full of Broadway gypsies made by choreographer Michael Bennett in 1974 and skillfully collated, with help from composer Marvin Hamlisch, lyricist Edward Kleban and playwrights Nicholas Dante and James Kirkwood, into the musical mainstay A Chorus Line. Next: a series of auditions for the show’s 2006 Broadway revival, through which, with odds of about 3,000 to 19, a brave new generation of spotlight seekers lived the Chorus Line experience — whether they later were anointed to dramatize it on stage or not. Ah, the theatah! The movie’s narrative strategy prudently obeys the musical’s classically counterintuitive formula — demystification as engine of enchantment — and the difference between this and American Idol is the obvious but crucial difference between documentary and idolatry.