Brother Mysterio-
The effect was good yes, but very hard to replicate. That is bad advice because if they tried exactly what you say they will most likely have poor results. What you are basically saying is have an actor at the end with good timing. That worked at the time but is most likely not repeatable.
I try to avoid the word "boo" mostly because that is what 10 year old girls say to scare people. She must have had great timing.
Allen H