I have some sometimes when I do nap. And I think It's a bit of fun, it's like a scarry movie but you re the protagonist. I feel that someone is coming from the stairs, to kill me, I feel his presence. And to break the dream, I know I have to move, but it's really hard, I have to focus all my will in moving my arm, I always succed to move my arm in the end so dunno what happens next.
Then I usually try to have some more rounds, by doing other nap. Once the first nap provoc a sleep paralysis the other next usually do too. The feeling is really strange it's more concious than a dream but you have wierd part like in dream.
About the falling thing: I 've read that's different. It is caused by some part of your brain "switching off" but area in control of your balance are not completly "switched off" so they re a bit buggy and give you the impression to fall.