When filmmakers talk about "the magic of cinema," we assume that they're talking about the medium's ability to transport us into fantastical worlds of wonder and imagination, or some sappy shit like that. What they really mean is that, like in any job, accidents tend to happen when they're shooting a movie ... and sometimes those accidents turn out to be far better than what the director originally had in mind and are left in the film.
6The Lord of the Rings Trilogy -- Viggo Mortensen's Pain Was Real
What Was Supposed to Happen:
Viggo Mortensen was a last-minute replacement to play Aragorn in The Lord of the Rings trilogy, which is kind of amazing considering that it's arguably the most important role. He pulled off the "risking life and limb to save the hobbits" thing like a champ, partially because he wasn't always acting.
For example, he actually murdered dozens of people over the course of those films.
The Happy Accident:
Due to a series of mishaps, miscalculations and God just straight up fucking with him, more often than not Mortensen really found himself in actual danger and/or anguish. For example, there's a scene in The Two Towers where Aragorn thinks that two of the hobbits are dead, angrily kicks a helmet and lets out a soul-scarring scream of grief.
However, that wasn't so much great acting as actual physical pain -- while kicking the helmet, Mortensen broke two toes and fell to his knees, not because the script required it, but probably because he simply couldn't stay on his feet anymore.
Lord of the Rings: Behind the Scenes
And that's why Aragorn is in a mobility scooter for the rest of the movie.
But hey, who needs toes? It's not like someone threw a knife at his face or something -- nope, that happened in The Fellowship of the Ring, when another actor misjudged a throw and accidentally flung a real knife at Mortensen's face at high velocity; according to the DVD commentary, Mortensen managed to swing his sword and block the knife, creating another unintentional badass moment for his character.
His role as a baseball player in that shitty Sandra Bullock movie saved his life.
Finally, while filming the scene in The Two Towers where an unconscious Aragorn is floating down the river, Viggo got caught in an undertow and nearly drowned due to the weight of his armor, barely escaping with his life.
Peter Jackson, of course, kept all these accidents in the films because they were simply the best takes. One of the key factors of the success of the trilogy was the fact that it just felt more authentic than fantasy movies tend to be, and having the actors almost dying and literally screaming in pain certainly helped in that sense.
5Being John Malkovich -- A Drunken Extra Attacks the Star
What Was Supposed to Happen:
In Being John Malkovich, a puppeteer called Craig Schwartz (John Cusack) finds a portal to actor John Malkovich's head and starts charging people to take the guy for a spin for 15 minutes. Eventually, Malkovich finds out what's going on, descends into the portal and ends up in what looks like the most fucked up Halloween party ever.
There are four Malkovich heads in this shot, but two are covered by a dress.
After leaving that place, Malkovich quite understandably freaks out and threatens Schwartz with litigation if he doesn't stop renting out his brain to strangers, storming off along the shoulder of the New Jersey turnpike.
The Happy Accident:
Drunk extras. According to the commentary by director Spike Jonze, some extras sneaked beer onto the set and got pleasantly plastered. The scene called for cars to just drive past Malkovich as he angrily walks away; however, one of the extras forgot that he wasn't a jock in an '80s high school comedy and yelled out "Hey Malkovich, think fast!" before lobbing a beer can at his head.
"Loved you in Dangerous Liaisons!"
Malkovich's surprised yelp of pain and subsequent cradling of his head amused Jonze so much that he decided to keep the scene in the film. This isn't so surprising when you consider that this is the man who helped bring Jackass to our screens, so we already know that he revels in the pain of others.
But it worked beautifully; rather than being a scene about how angry John Malkovich is at the main character, it became a scene about how his life is falling apart around him. It helped us empathize with the actor, not to mention that the moment was also very much in line with the absurd humor of the screenplay (how did the guy even know it was Malkovich?).
And how did Malkovich fit into that dress?
In regards to the drunken extra, this inspired moment of sheer douchebaggery paid off in dividends -- since he now had a line in the movie, his pay rate was bumped up to $700 a day, that of a speaking role, and he was also now eligible to get his Screen Actors Guild card, which just goes to show that being a complete and utter dick is the only way to get ahead in Hollywood.