When NBC’s SNL ushered in a new format of bawdy and topical sketch comedy nearly a half century ago, it arrived in the world via a most improbable birth. The 90 or so tense minutes leading up to the live pilot episode are the subject of Jason Reitman’s punchy, energetic ensemble dramedy Saturday Night (B+). Gabriel LaBelle masterfully plays optimistic show runner Lorne Michaels opposite an array of splendid fellow writers (a whip-smart Rachel Sennott and deadpan-droll Tommy Dewey are fantastic) and legendary on-air comedians (really solid impressionistic impersonations by Cory Michael Smith as Chevy Chase, Dylan O’Brien as Dan Aykroyd and Lamorne Morris as Garrett Morris). The movie crackles with kinetic energy as last-minute wardrobe fitting, set building, camera testing and script doctoring activities collide with the madcap antics of these delirious denizens of the famed 30 Rockefeller Plaza studio. Reitman justifiably ribs the old guard of classic TV production in the form of a smarmy executive (Willem Dafoe) and a really dickish Milton Berle (J.K. Simmons) as he curates a near real-time portrait of art and anarchy in the making. The film is consistently entertaining, even if a bit lightweight in terms of story or substance. It’s a whirling dervish energy fueled showcase of how unusual and disparate ingredients collide like fermenting hops in a creative home-brew. The film offers the thrills of artists working at the top of their game in a pressure cooker, and it’s a bright return to form for Reitman who cracks open what seems like a familiar vault and unleashes a vibe of his own. The inventive comic syncopations he puts into motion will tickle and tantalize.