I went to the east coast to visit my sister and her husband. They lived in a narrow, but very tall house (connected to 4 other houses in a single building, not sure exactly what the word is) and I was going to be staying on the 4th floor. I got in pretty late, so my brother-in-law stayed up to meet me and show me to my room while my sister was already asleep.
I went to bed and laid awake for about an hour like I do every night. I then hear footsteps coming up the stairs leading into my room and I assume it's my sister (I was at least 1/4 asleep, otherwise that would have seemed a little odd to me) coming to say welcome and goodnight. I decide I'm going to pretend to be asleep, then scare her when she gets close enough.
I close my eyes allowing in just enough light for me to see, and sure enough, my sister shows up in my doorway. She walks up to the edge of my bed, then says, "Huh, I guess he's already asleep" out loud to herself. She then pulls out her phone and points it at me as if she's going to take my picture (even at 1/4 asleep, this seemed very odd to me), so I look up at her face now illuminated by her phone and her facial expressions are changing instantly in 1/2 second intervals. She's calmly looking at her phone for 1/2 a second, then instantly staring directly at my eyes with a wide cartoonish smile for 1/2 a second, then BOOM frowning at the ceiling, BOOM calmly looking back at me, etc. These changes were instant, humanly impossible, like flipping through pictures of her face.
This freaked me out, and that's when I realized I couldn't move my body. I was now looking at the ceiling paralyzed and trying to shake myself free with all my might, until finally I shook hard enough and snapped out of it. I sat up and looked over, and she was gone. There was no change of awareness, no sense of waking up. I felt completely alert and awake throughout the entire experience. I couldn't fall asleep for hours after that.
This was my first experience with anything like that. Of course, the next morning I asked her if she was in my room the night before and she said no.
tl;dr I had the first of what would end up being many bouts with sleep paralysis. This was the scariest, especially because it was my first, but the small figure crawling up my legs towards my chest a few years later gives it a run for its money.