If you attend “The Rocky Horror Show” as a mere spectator, you’re missing half the fun. On cue throughout the musical mayhem at Gateway Playhouse, the audience throws nonlethal objects toward the stage. My personal favorites: “A toast!” as cross-dressing host Frank-N-Furter raises a glass. The audience hurls toasted bread — preferably unbuttered. It’s topped only by Brad shouting “Great Scott!” as Dr. Scott is wheeled in. Rolls of toilet paper — we couldn’t tell if the brand was Scott’s — are sent streaming from near and far reaches and every corner of the theater.

For novices — “virgins” in “Rocky Horror” cult parlance — a primer: Brad and Janet become engaged in the opening friends-of-the-bride wedding scene. (The audience throws rice, of course.) After their car breaks down in a thunderstorm, the couple find themselves inside the castle of a demented “scientist” celebrating his creation — far prettier than that of Frankenstein, perhaps Frank-N-Furter’s neighbor in their native Transylvania. The host and his minions make it their project to teach the virginal guests to walk on the wild side, mostly in their undies. Frank-N-Furter’s are more glam — costumes by Trent Pcenicni — paraded on Brittany Loesch’s cavernous set. Matthew LaBanca wears it well in retro Tim Curry (the original Frank) glory. And sells it, too.

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Usherette Lisa Karlin gets the party going with “Science Fiction Double Feature” — a reference to “Rocky Horror Picture Show,” the 1975 movie that spawned the cult following. Karlin doubles as maid Magenta, whose duet with butler Riff-Raff (Courter Simmons) leads the show’s dance-lesson highlight, “The Time Warp,” accompanied by Andrew Austin’s rocking band (kitsch book, music and lyrics by Richard O’Brien). Hunky Josh Canfield as the title character, strips down nicely as Frank-N-Furter’s love-object creature.

Brad (Harley Jay) and Janet (Emily Behny), appealingly comic in their innocence, are targets of rowdy audience epithets. Their silhouetted sex scenes involving Frank-N-Furter turns “Touch-a-Touch-a-Touch Me” into an R-rated hoot. Mychal Phillips as Frank-N-Furter groupie Columbia torches the floor in lovelorn song and tap-dance, while Justin Colombo as Meatloaf-inspired Eddie and his Strangelovian uncle ushers in the “double feature’s” sci-fi portion. Geoffrey Owens, formerly Elvin on “The Cosby Show,” spoofs ironically with “Masterpiece Theatre”-style summations.

Director-choreographer Keith Andrews throws a “Rocky Horror” party. (The midnight show Sept. 9 is sure to be raucous.) Come with your toys or buy a kit of harmless projectiles ($10) from friendly “phantom” actors.