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.