Rule No. 6 at Midnight Madness: Don’t have sex.  It’s a fairly crowded theater, lit well enough that this should be a given, but with The Rocky Horror Picture Show, nothing can be assumed.

“Those rules exist for a reason,” says Chris Siuty, a cast member for the past ten years.

Looking at the audience, there’s no reason for him to elaborate. About half are undressed for the lingerie contest set to take place before the movie. The room is filled with shiny underpants and underage breasts, leather and leashes in quantities that would make most people blush. Gender lines don’t exist. Decorum doesn’t exist. There is no dress code in the Music Box Theater tonight.

This is Midnight Madness, a live performance of The Rocky Horror Picture Show. The group has been performing what is called a “shadow cast” to the movie for the past 20 years.  As the original film of The Rocky Horror Picture Show plays on the screen, the cast acts out the entire movie, lip-synching along.

At the beginning of every show the “virgins” – people who have never seen the movie in a theater before – get painted with red Vs and are put through various embarrassing tasks. Then a couple cast members go through the rules: namely, no fire, no drugs and no sex.

“We’re going to fucking offend you and there’s nothing you can do about it,” Dave, a cast member, yells to the audience.  The audience cheers back.

When the movie starts, there is no hush, just screams and jeers. Various projectiles fly through the air on cue as part of the audience participation that includes a variety of choreographed reactions timed to moments in the the movie. Snap the balloon now. Throw the toilet paper now. This defines cult classic.

The people here have day jobs, marriages and regular lives, but three times a month they don high heels and fishnets and lose their inhibitions. Performing won’t get them money or fame; it’s a love of the movie that keeps the cast coming out two weekends a month.

“We survive completely on donations and the sale of the virgin [prop] kits,” says Jeff Nurkiewicz, the 39-year-old engineer who leads the cast. He began attending midnight performances of The Rocky Horror Picture Show almost every weekend in 1987, and finally joined the cast in 1995.

Nurkiewicz talks about breaking his ankle falling off of the stage, and how he’s getting too old to keep performing in Rocky, but then points to Elaine, known on the cast as Mom, who performs in the cast with her two grown sons.

“As long as she can keep coming to the show, I think I can keep time warping,” Nurkiewicz says.

Midnight Madness will perform The Rocky Horror Picture Show both at their “Ides of March Pre-St. Paddy’s Day Show of Doom” on March 15 at the Wilmette Theater and their “Post St. Pat’s Hangover Cure Shows” on March 21 and 22 at the Music Box Theater.  For a complete listing of their performances, visit their Web site.