First and most funny, is that Terry Gilliam started out directing this film, and due to a very mutual disagreement between he and the cast, decided to walk off set and take a nap under a nearby tree.
Terry Jones took up the directorial mantle from that point onward. Which is why they both have director credits.
Reportedly the UK band Pink Floyd was so enthralled by MP's humor that they donated proceeds from their "Dark Side of the Moon" album to help pay for the filming of the "Holy Grail"!
The inspiration for the "Killer Rabbit" scene in the film is actually based in some artistic reality. In several churches through out England, " A knight running away from a rabbit" is actually a symbolic device used to denote cowardice! Notre Dame has three medallions with this theme as well.
The scene with the "Black Knight" was initially played by John Cleese, but then after his first leg gets cut off, Cleese was replaced by a one legged silver smith/ local actor. Which helped greatly for the no-legged scene as they only had to dig a small hole for his one leg.
The representation of God in the film was actually a photo of a 19th century cricketer W.G. Grace.
Graham Chapman, unbeknownst to his fellow cast mates was suffering greatly due to his alcoholism, and during the filming of the "Bridge of Death" scene had to be replaced by the 1st A.D. for the shot. Chapman was suffering from the DT's and since they were in a remote filming location, there was no alcohol around for miles!
And lastly due to refusal by the Scottish Department of the Environment, permission was denied to shoot at many of the chosen locations, so most of the varying castle shots were all filmed at Doune Castle. The closing scenes at Castle Aargg were shot at the recently rebuilt Castle Stalker.
I can't imagine there are too many folks out there that have not seen this hilarious classic. But if there is I highly recommend you watch this film and more succinctly buy the DVD. Make sure you get the Special Anniversary Edition. There are oodles and scads of extras on it. Terry Jones and Michael Palin revisit many of the filming locations, recant tales from the experinece of getting some of the more dangerous shots, and share private off screen jokes.
There are other really cool features like the lyrics to all the songs, great interviews with the cast, some extra Terry Gilliam goodies and a whole lot more. If you are an avid collector or just a fan of Monty Python, this is a must own!