Okay, I've encountered what I consider strange usage of participles in a movie: The Incredibles.
In several places, where, in English we would say "Be seated" (a command), the characters look at another and appear to say simply "sentado." The mom (Elastigirl) says this to her kids, and Bob's (Mister Incredible's) boss says it to Bob once. I would have thought it should be said "¡Sientate!"
In another place, Bob's boss says (as Bob is leaving to stop a mugging in progress), "¡Detente ahí o despedido!" which means "Stop there or you're fired!" I would have thought it should be said "¡Detente ahí o estás despedido!"
Both of these can be found in a scene between Bob and his boss between 00:27:23 and 00:29:10...English version / Spanish sound track...
Did I hear it right or wrong?
And, if I heard it right, is there some grammatical rule that covers such uses of participles?