I can’t believe no one has mentioned Friday the 13th.
I don’t even have to describe the scene.
Ah, gotcha. I didn’t remember that one as startling because the movie had been semi-spoiled for me already when I saw it, since I knew going in that it was a horror movie. Up to that point, I was just waiting for it to all go south, so wasn’t surprised when it actually did.
A friend told me she saw it at a theater, not knowing it was a horror movie, and the scenes from the girl’s apartment were a total shock. (She wasn’t as scarred by it as I was, though, since at random quiet moments she’ll start saying “kee kee kee kee kee” in a really high-pitched voice, knowing that the movie genuinely disturbed me and just the sound of it gives me the heebie-jeebies.)
Well, the first time I saw it, I didn’t perceive any of those as flashbacks but instead as visions. My initial take on the movie was that it was pointless horror all done for shock value, but this thread (note: thread has open unboxed spoilers for “Audition”) changed my mind. I still don’t like it, and will never actually watch it again voluntarily, but at least I can see that it did have a higher message.
And if you take that interpretation of it – that the main character was a misogynistic jerk who was casting for the role of an ideal woman instead of actually meeting someone and getting to know her – then the “flashbacks” make even more sense. He wasn’t remembering stuff that he’d witnessed, but he wasn’t having psychic/ghostly “visions” either. Instead, he was gradually piecing together the girl’s story, remembering bits he’d overheard from her or other people, but had ignored because of his own self-obsession, and finally realizing just how horrible the story really was.
I’m with fusoya about the cat in Alien. What was his name? Jonesey?
From Twilight Zone - The Movie - “Want to see something really scary?”
That is the reason we don’t let my mom hold the popcorn … we were watching that movie, and that moment came-- popcorn everywhere!
I inherited my mom’s jumpiness, much to the delight of my SO.
For one of the guys I went to see Aliens with the second time, it would be the moment when the facehugger in the specimen jar suddenly jumps against the glass. In this case it was scarier, because the rat who’d seen it the previous night chose that exact moment to grab him near the floating ribs. 
My first thought was the end of Carrie but my favourite “slump into your seat moment” was in Monster’s Ball when Heath Ledger’s character kills himself. I didn’t even know he was in the movie and was settling in to the story when…
The Head.
Alien…Dallas in the Jeffrey tubes…
I saw Jaws when it originally came out. In a theater in Florida. When the severed head rolled into view in the hole in the boat, everyone jumped (and screammmmmmmmmmmmmmmmmmed). A few rows ahead of me a bucket of popcorn flew up and almost hit the ceiling.
Meet Joe Black:
When Brad Pitts character (Joe Black) gets hit by the cars.
In The Omen when… Gregory Peck was digging through his son’s hair, finds the “666” and that creepy satanic nurse lands on his back. A guy I worked with said the woman sitting behind him at the theater had obviously seen it before and at that point in the movie buried her fingernails into his back. He said he damn near flew into the screen.
In Predator when… Jesse Ventura is chasing something through the jungle and sees some little animal, smiles, says "It’s just a little something and then WHAM! get his innards blown out by the Predator. I remember demoing our home theater for some friends and at that scene the wife screamed and red wine went everywhere.
In Final Destination when…
Alex and the jerkish guy are arguing. The jerkish guys girlfriend loses it and backs away right into the path of a speeding bus.
In Arachnaphobia
Near the opening scene after they sprayed the tree with insecticide, all these critters start falling out of the tree. A guy taking pictures go’s to take a picture of a seemingly dead spider, only to have it jump right at him/the screen
Oh yeah, that was a major one too.
That movie was shown, for some odd reason, in my 6th grade class (when I was in sixth grade, that is), and everyone jumped.
In The Crying Game, when
Forrest Whittaker’s character gets run over by the tank of the rescuing British soldiers. I had come to like that character and thought he’d be there for the whole movie. It was completely unexpected and shocking - and it looked absolutely real.
Can you elaborate in a spoiler tag? I tried to find a good description of what happens online and can only get the vaguest hints, but it seems to be frequently mentioned as one of the best horror scenes ever.
When Bilbo reached for the Ring in Fellowship of the Ring.
Mittens
Ha! For shivers, my son and I replayed the Bilbo scene over and over again.
My big jumpy scene is from an old movie, The Changeling. (Based on a ‘true’ story that happened here in Denver.) We saw it in the theater years ago, and rent it every now and then.
This movie gets a bit hokey at the end, but the buildup is very scary indeed.
Trying for a new start, a man (George C Scott) looks into purchasing the proverbial haunted house. As he stands there deciding to buy it, a teeny-tiny window high on the third or fourth floor shatters into pieces.
Many in the theater gasped and jumped in their seats, including my usually stoic, immovable object husband.