(Warning: Here there be spoilers!)
The term suspension of disbelief was coined by poet Samuel Taylor Coleridge in 1817. It is used of the individual's willingness to accept the basic assumptions of a work of fiction as real. More simply put, it is refers to the person's willingness to suspend disbelief in a work of fiction. Suspension of disbelief is necessary for the success of movies as much as any other medium. If a film fails to convince the viewer of the reality of its world, then that film has failed for the viewer.
Of course, that in which one is willing to suspend his or her disbelief is going to vary from individual to individual. Having raised on a steady diet of comic books, old pulp magazine novels, and fantasy movies, I am more than willing to believe that a man can fly or that Hyrda's teeth when sown will turn into an army of skeleton warriors. On the other hand, my mother was not--it is for this reason she hated science fiction and fantasy movies. It is then not necessary for any film to suspend the disbelief of every single person in the world, as this is probably not possible. But then, a movie must be able to suspend the disbelief of its prospective audience to succeed.
Suspension of disbelief will also vary according to individuals based on their knowledge of any particular subject. This is especially true of period pieces. What might convince the ordinary person might not convince an expert historian. Being a history buff myself, I have had a problem with a few movies in this regard. One of these is The Da Vinci Code. Not being Christian, I have no theological objections to the idea that Jesus could have married and had children. That having been said, I have a difficult time believing that if he did, we would have heard about them by now. Granted, we know little about Jesus's life, but a wife and kids would have been a hard secret to keep for centuries. A bigger problem I have with the premises of The Da Vinci Code is that it supposes the Merovingian line nearly died out (for those who do don't know who the Merovingians were, they were a Salian Frank dynasty who ruled what would become France and parts of Germany from fifth to eighth centuries. Now as any good genealogists knows, the Merovingian line did not die out. In fact, their descendants probably number in the millions (I am one myself)! While I might be able to accept the idea of Jesus having offspring, I cannot accept the idea that the Merovingians nearly went extinct! Of course, The Da Vinci Code has nothing on Braveheart when it comes to historical inaccuracy--its inaccuracies are so glaring and so plentiful (and some amount to outright slander) that I actively loathe that film.
Of course, these films are enjoyed by many people who don't know much about history. But I can think of other instances in which something in the movie is so outlandish or unbelievable, that it ruins the whole movie for people. A perfect example of this for me is the Bond film Die Another Day. In my humble opinion, Die Another Day could have been one of the better Bond movies. It featured one of the better Bond villains (Toby Stephens as Gustav Graves) and one of the best Bond girls (Halle Berry as Jinx). That having been said, I have one major problem with the movie that ruins it for me. As bit of a Bond connoisseur, I have always thought that the gadgets 007 uses should have some basis in reality. After all, for me the Bond series is not science fiction or fantasy, but a series of exaggerated spy dramas. What ruined the movie for me was the invisible car. In an episode of The Man From U.N.C.L.E. I might be willing to accept such a gadget (it was, after all, science fiction), but this is Bond--the gadgets and the plots have to have a little basis in reality. Given that invisibility is impossible with today's technology--I suspect it may be impossible, period, the invisible car made me unwilling to suspend my disbelief in Die Another Day. And I am not alone. Bond fans all over the world agree the invisible car is the worst gadget in the history of the series.
Another example of a movie which ultimately failed to suspend my disbelief is The Shadow. Now I was predisposed to really like this movie. I have loved the character of The Shadow since childhood. I have read many of the pulp novels and listened to episodes of the radio show. And this film looked like it would be good. I thought the casting was perfect--Alec Baldwin as The Shadow and John Lone as his archenemy Shiwan Khan. Indeed, I must say I enjoyed most of the film, probably the first three fourths of it, but then the movie postulated something so outlandish that it destroyed my willingness to suspend disbelief. In the film, not only does The Shadow have the power to cloud men's minds, but so does Shiwan Khan. The movie then has the audacity to claim that for literally years Shiwan Khan has used his ability to control the minds of others to cloak his skyscraper headquarters from view--quite simply, he virtually made the building invisible. I have several problems with accepting this idea. First, Shiwan Khan would have to have been nearly godlike in power to have masked a building for years without its discovery, especially when he would have been busy doing other things as well (controlling his crime interests, plotting to conquer the world, eating, sleeping). If Shiwan Khan was this powerful, then why didn't he simply use his powers to mind control millions of people to become his willing servants? It would seem to be a lot quicker route to world conquest than atom bombs and invisible skyscrapers! Second, like The Shadow, Shiwan is only able to cloud men's minds and make them "think" they don't see something--it isn't really invisible. While Shiwan Khan's skyscraper may not have been seen by the human eye (or more accurately, the human mind), it could still be captured by film cameras and still picture cameras, both of which were plentiful in the Thirties. At some point in the first few months someone would have surely taken a picture or filmed the vacant lot, the picture or movie of which would reveal it wasn't vacant at all! Third, where the skyscraper was supposed to be appeared to the human mind to be an empty lot. It would seem likely that at some point someone would have literally stumbled into the building. It could have been as simple as kids wanting to play ball in the lot or a wino looking for a place to sleep, but at some point someone would have tried walking into the lot only to bump into a very solid, if unseen, building. The building was invisible to the human mind, but presumably not to its physical presence--it could still be touched and felt! While The Shadow could have been an enjoyable film for me, the invisible building ruined it for me.
These are both examples of films that, for me at least, failed to convince me to suspend my disbelief. There are many other examples. From the above examples one can see that even one little thing can destroy suspension in disbelief. This is why sceenwriters and directors must insure that everything within their film is realistic to the milieu they are portraying. Those making period pieces must make sure that everything is historically accurate. Those making sci-fi, superhero, and fantasy movies must insure that everything they portray fits the premises of the milieus they are portraying. In fact, I think suspension of disbelief may be more difficult on film than it is in literature or any other medium save perhaps television. Because movies can capture reality on film or even simulate reality on film, I suspect the average viewer demands much more realism from them than they might, say, a book or a graphic novel. It is then much easier for a director or screenwriter to make a misstep that could result in the movie failing to suspend the viewer's disbelief. When that happens, the entire movie is usually ruined for the viewer.