Reasons the OP thinks these questions don't fit the site:
It has nothing to do with the film itself.
False. Audience participation is the reason for the film’s success and longevity, and has been since 1975, when it was first released.
When the Library of Congress selected Rocky Horror Picture Show for preservation as a culturally significant film, it said:
"The Rocky Horror Picture Show" is the quintessential midnight movie, offering a participatory moviegoing experience to millions of college students and other fans of this cult classic of American cinema.
Library of Congress
The ultimate "midnight movie," "Rocky Horror" revolutionized prevailing notions of audience participation during film screenings.
- Library of Congress
Audiences were singing along in movie theaters during the musical numbers during the initial theater run of the film in 1975, as documented at the United Artists Theater in Westwood, California.
“Shadowcasts” (i.e., people acting out the movie in front of the theater while the film was playing on the screen above them) began in New York’s Waverly Theatre in 1976, with Sal Piro and Dori Hartley leading the cast.
“Call-backs” (i.e., shouting at the screen) began during the second (still ongoing) theatrical run which began in 1976; it is said that the first person to do call-backs was Louis Farese Jr., at the Waverly Theatre in New York City, on Labor Day, 1976. Farese’s witticism was the first piece of the audience call-back repertoire which soon became standard at future screenings.
The midnight screening phenomenon was not mere happenstance. Twentieth Century Fox executive Tim Deegan arranged the first midnight screenings, beginning on April 1, 1976, a fact that Roger Ebert mentioned:
When the film was first released in 1975 it was ignored by pretty much everyone, including the future fanatics who would eventually count the hundreds of times they'd seen it. "Rocky Horror" opened, closed, and would have been forgotten had it not been for the inspiration of a low-level 20th Century-Fox executive who talked his superiors into testing it as a midnight cult movie.
These first midnight screenings took place in New York City; Fresno, Merced, Sacramento, and Modesto, California; New Orleans, Louisiana; Chicago, Illinois; and Pittsburgh, Pennsylvania.
Rocky Horror was not only found in the larger cities but throughout the United States where many attendees would get in free if they arrived in costume.... Before long, nearly every screening of the film was accompanied by a live fan cast.
References to RHPS in popular culture almost always focus on the audience participation aspect of the film, not the film itself. Charles in Charge devoted an episode to one of the characters (played by Nicole Eggert) going to a midnight screening in costume. The Drew Carey Show had an epsiode about the characters taking a road trip (in costume) to see RHPS in New York. The Rocky Horror Wiki lists 25 other television shows with significant references to RHPS, and 38 shows with minor references. Again, most of the references are directed at the audience participation aspect rather than the movie itself.
In short, audience participation is the main reason the movie is noteworthy.
It's a [sic] off-topic, primarily opinion based, list question, for some things that may happen at some but not all viewings.
Not true, in my opinion. It is on topic, because if you see RHPS in the theater, you are virtually certain to see some or all of the things mentioned in the answers. The only way you can watch RHPS and not experience audience participation is to watch it on video in a private location.
Primarily Opinion Based
Not really true. Audience participation is well documented in countries around the world. The BBC reports on it from Britain.
Such traits can be found among followers of The Rocky Horror Picture Show, where fans ritually dress up, sing and act out scenes, and shout out during screenings. In fact audience participation is credited as a major factor in keeping Rocky Horror alive at midnight screenings.
And in the US, Roger Ebert was more impressed by the audience than the movie.
"The Rocky Horror Picture Show" is not so much a movie as more of a long-running social phenomenon... Inside the theater, the fans put on a better show than anything on the screen. They knew the film by heart, chanted all of the lines in unison, sang along with the songs, did dances on stage, added their own unprintable additions to the screenplay, and went through a lot of props like toilet paper and water pistols. They also formed a sort of weird extended family. They met every week, exchanged ritual greetings, celebrated each other's birthdays and other major holidays, and even dated and married and gave birth to a new generation of "Rocky Horror" cultists.
The Popular Culture Association of Australia and New Zealand published a journal article on the subject in 2014, written by Renee Middlemost of the University of Wollongong.
Screenings of this film have established standards for organizing cult film events as well as ritualized audience participation.
The official fan site lists late night audience participation showings in 28 states plus the District of Columbia in the US, as well as in Canada, Australia, France, Germany, Italy, Israel, Mexico, New Zealand, and Spain; in addition, there are (or have been) screenings in Japan, South Africa, Argentina, Britain, Austria, Ireland, and many other countries.
There is no opinion involved - the question is asking "what happens and why?", not "Is it okay that these things happen?"
Not really true.
Although the title is:
What happens in a Rocky Horror Picture Show showing and why?
The actual question text is more limited:
I have never been to a showing of the movie, nor have I even seen it. I have heard that it is quite the experience. People dress up, yell things at certain parts of the movie, and even throw toast?!
My question is why is this done? Is there a reason or significance for the actions during a viewing?
There is no request for a list here, only an explanation of why audience participation takes place at RHPS screenings.
Even if we judge the question based on the title alone, the “list” here is finite and acceptable by community standards. See for instance this meta question and answers.
But again, it isn’t a list question, it simply has one answer that uses a list.
About some things that may happen at some but not all viewings.
No, as the answers make clear, it would be almost impossible to find a screening at which no participation takes place; the call-back lines will vary a bit, and some props may be forbidden at some venues, but the overall experience doesn’t vary much.
The guidelines for audience participation vary little from country to country; it is far more common for differences between two theaters' audience participation systems to vary because one of the theaters won't allow rice throwing or open flames.
Will this set precedent for less useful questions about what people do while watching other movies?
I don’t see how.
The OP mentions concerns about RHPS screening questions leading to far less desirable questions:
Would a question about X-Men cosplay, or how to plan a Harry Potter marathon, or what alcoholic drinks and rules are best for getting plastered at a LOTR drinking game or what happens during a Super Bowl viewing party be on topic?
...I would love to know what's the best Harry Potter drinking game.
These comparisons are just silly.
Off-topic because it is about clothing design. People usually engage in cosplay at conventions, not screenings. Yes, people dress up to go to some movie premieres, including X-Men, but it isn’t particularly common, and there isn’t a 40-year-long tradition of people dressing up like Wolverine and acting out X-Men: The First Class in the theater every week. The Library of Congress has never selected an X-Men movie for preservation based on the well-known, decades-long history of in-theater audience participation.
And obviously, no one is suggesting that “How do I make a Frank N. Furter costume?” would be on topic here.
Planning a Harry Potter marathon; Best alcoholic drinks and rules for getting plastered at a LOTR drinking game; What happens during a Super Bowl viewing party; Best Harry Potter drinking game
Again, no tradition, no universal or nearly-universal practices in multiple countries for several decades, etc. This is the definition of an opinion based question, because there is no established tradition. There are some things that happen at most Super Bowl parties - drinking, eating, and watching football - but these are too obvious to be a good question with an objetive answer. “The best” drinks, rules, and drinking games will obviously vary by individual preference. Drinking games and alcoholic beverages aren’t on topic here; movies are.
Back to the thrust of the original question: “Are questions about viewing parties on topic?”
I wouldn’t call RHPS screenings “viewing parties”, because “viewing parties” might be interpreted as “watching a movie with friends”, which is not on topic.
What we’re talking about, with Rocky Horror Picture Show, is a well-known, long-standing tradition that has spread across at least sixteen countries on six continents, and the in-theater activities in those locations follow a set pattern. Go to RHPS in New York, then in Tokyo, then in Buenos Aires, then in Brisbane, then in Toronto, then in Dublin. The experience you’ll have in each of those places will be very much like the experience you had in each of the others.
So are questions about the in-theater audience participation at RHPS screenings on topic? Absolutely. Audience participation is the reason the movie has been in theaters continuously since 1975; it is why the movie was honored by the Library of Congress; it is why the movie is still remembered today.
In short, as Roger Ebert put it:
The whole thing about "Rocky Horror" was that the movie played as a backdrop to the stage show by the fans.
The movie is the backdrop, the theater experience is the point. The two cannot, and should not, be divorced from one another. I can't think of another film that is so intimately associated with a near-universal system of audience participation in over a dozen countries on six continents, so I don't see it becoming an issue for any other movies. If it does, we should deal with each instance on a case-by-case basis.