"The kid to freak out the extremely evil dude will drop in at the Hufflepuffs' summer party..."
Dumbledore: eh?
Snape: Oh man...that will not be easy to explain...
Voldemort: I'm truly scared now...Severus, here, an Aspirin, and if you ever drink again while you're on duty, it's Crucio!
Yes, the choice of words is important. *g*
I'd say who, or better, what chooses the words is...what causes the prophecy. But what exactly is it that makes somebody fall in trance and make a prophecy? Fate? Buddha? God? Marlboro, or too many mushrooms on your pizza?
The state Trelawney falls into when she makes a prophecy is somewhat like that of Ginny when she was possessed by Tom Riddle...trance, no memory, different speech patterns...so I guess Trelawney's possessed, too, in one way or another. Maybe somebody wanted to play fate and imperioed her...but no, that's unlikely. But where does it come from?