Filmmaking is such a chaotic, haphazard process that it's nearly impossible for the people involved to tell if the movie is going to be good or not. Scenes are filmed out of order, much of what you shoot is going to get cut, and you have to trust that whatever effects fill in the big green wall behind you are going to look good.
So maybe it's not all that surprising that some of the stars of your favorite movies had no goddamned idea what film they were even making. For example ...
6 Carrie (1976) -- Piper Laurie Thought She Was Filming a Comedy
Carrie is the story of a horribly abused high school outcast with supreme telekinetic power who is finally pushed too far by a prank on prom night and decides to use her mutant ability to kill every single person who ever looked at her askance. To prove she isn't wholly evil, she also kills John Travolta as a public service.
If she had just stopped there, the town would have probably been ready to call it even.
Then Carrie stumbles back home and kills the real villain of the movie -- her mother, a cruelly evil Bible-thumping taskmistress who was Carrie's chief tormenter -- before using her mind-flexes to implode their house.
Carrie's mother is so over-the-top evil that she's like a kindergartner's drawing of violent religious insanity. In fact, the character was so broad that Piper Laurie, the actress who plays her, was convinced the movie was a comedy when she first read the script. The mother was so operatic to her that she thought the character was intentionally cartoonish.
She was surprisingly lax about wire hangers.
Director Brian De Palma had to take her aside and explain that he wasn't exactly going for laughs with his film about abuse, rage, and supernatural murder. Still, Laurie couldn't shake the idea that her character was completely ridiculous, and she laughed constantly between takes. This might have something to do with the scene in which she refers to her daughter's breasts as "dirty pillows," completely stone-faced and without a trace of irony, or it could be related to the Phantom of the Opera cape she wears in every scene with a similar lack of elbow-nudging winks.
Whatever the case, Laurie still insists that Carrie is really a black comedy. We are curious to see what constitutes gut-busting hijinks in her mind, because she's fucking terrifying in that movie.
"I'm a horrifying monster! Get it?!"