The catchphrase part is easy. People have been quoting the film (and everything else produced by Monty Python's Flying Circus) to each other since the Holy Grail premiered in 1975. In some circles, out-quoting Python films and skits has become a survival of the geekiest. We routinely win. Not because we know more lines than our fellow competitors, but because we're female and therefore are an anomaly to many who play the Flying Circus games. But we digress. The most important shout-out in our Holy Grail interactive movie experience is "Get on with it." Everyone else--even God Himself--shouts "Get on with it" during tangents, the long bits and the boring parts. You should, too. Next, for each mention of Camelot, holler, "It's only a model." Every sighting of an injured peasant, knight, party guest or other character should be met with "Just a flesh wound." "I'm not dead" is an acceptable substitute. Advanced participators can even try the more difficult moments such as Tim the Enchanter's "death awaits you all with nasty, big, pointy teeth" and Dennis the peasant's "Help, help, I'm being repressed."
Finding viable props and costumes should be as easy as defeating the Black Knight. Dress as the Knights of the Round Table, peasants, the desperate virgins of Castle Anthrax or even the English police officers who ruin the epic Braveheart-like ending by arresting Arthur and Bedevere when the grail is finally within reach. As for props, the most important are the coconuts, which you'll use to make horse galloping sounds because every knight needs a horse, even if that horse is a man with a backpack hitting two halves of coconuts together and naying occasionally. Other suggestions include Spam (a Python requirement), shrubbery (to appease the Knights Who Say Ni), tiny plastic or rubber cows (to recreate the cow-catapulting scene) and bananas (how the world is shaped, according to Bedevere). As soon as we get this completely mapped out, we'll begin to work on our interactive version of Mel Brooks' Young Frankenstein. For now, enjoy Monty Python and the Holy Grail by gathering up your autonomous collective and seeing it on the big screen.