
The cult following The Rocky Horror Picture Show (1975), or audience participation, is a cultural phenomenon surrounding the large fan base of enthusiastic participants of the movie generally credited as being the best-known cinematic "midnight movie", if not the first. The film gained popularity because of fan participation as much as anything else. "Shadow Casts" of fans acting out the entire movie below, or in some cases directly in front of the screen, are almost always present at showings.
During a showing, ad-lib responses, more commonly known as call backs, are lines the audience may shout out in response to events occurring on screen, as a form of audience participation. In some venues, audience members who provide incorrect or poorly timed responses may find themselves angrily shouted down just as if they were being disruptive in a normal movie. However, creative new lines are usually applauded and even added to the local repertoire. There have been audience participation albums recorded and scripts published. However, most fans feel that it is preferable for responses to grow organically from the local culture.
History
The film came about due to the tremendous success of the stage musical and opened in the United States at the United Artists Theater in Westwood, Los Angeles, California, on September 26, 1975. Although the theater was selling out every night, it was noted that many of the same people were returning to see the movie. This situation on LA turned out to be an exception, not the rule as it was not doing well elsewhere in the US.

A part of audience reception can be recreating the art. This is how the fandom of Rocky Horror developed into a standardized ritual. The performances of the audience were scripted and actively discouraged improvising, being conformist in a similar way to the repressed characters. Rocky Horror helped shape conditions of cult film's transition from art-house to grind-house style.

According to J. Hoberman, author of Midnight Movies, it was after five months into the film's midnight run when lines began to be yelled by the audience. The first person to yell out an audience participation line during a screening was Louis Farese Jr., a normally quiet teacher who, upon seeing the character Janet place a newspaper over her head to protect herself from rain, yelled, "Buy an umbrella you cheap bitch". This self-proclaimed "counter point dialogue" was soon helped into standardization by Piro and repeated nearly verbatim at each screening. By that Halloween, people were attending in costume and talking back to the screen. By the end of 1979, there were twice-weekly showings at over 230 theatres.
The National Fan Club began in 1977 and would merge with the International Fan Club; the fan publication The Transylvanian printed a few issues. A semi-regular poster magazine was published as well as an official magazine.

In San Francisco Rocky Horror would move from one location to the Strand Theatre located near the Tenderloin on Market Street. The performance group there would act out and perform almost the entire film, unlike the New York cast at that time. The Strand cast was put together from former members of the Berkeley group, disbanded due to less than enthusiastic management. The Strand group had performed at two large science fiction conventions, in Los Angeles and San Francisco, and were offered a spot at The Mabuhay, a local punk club; and even performed for children's television in Argentina.
Audience participation also includes dancing the Time Warp along with the film, and throwing objects such as toast, water, toilet paper, hot dogs, and rice at appropriate points in the movie. Many theatres forbid throwing items that are difficult to clean up. In many cases, a total ban on throwing objects has been instituted due to severe damage to movie screens. Fans often attend shows in costume as the characters. At a now-defunct theater in New Orleans, the local "Eddie" would ride his motorcycle down the aisle during Meat Loaf's/Eddie's song, "Hot Patootie."

Call backs
If you want a read an example of a call back script, you can read it here.