The Gemara suggests that Rebbi Meir allows a person to be a wall for a Sukah since he is not a Kosher Mechitzah. The Gemara rejects this because Rebbi Meir only invalidates an animal as a Mechitzah, not a man. As Rashi explains, the reason a man is a valid Mechitzah is because we are not afraid a man will run away.
But that is only true if the man knows that he is a Mechitzah. The Gemara concludes that it is permitted to use a person as a Mechitzah when he does not know that he is being used as a Mechitzah. In such a case, he certainly might run away in middle. Why, then, didn't the Gemara answer that Rebbi Meir invalidates the man in the Beraisa to be used as a Mechitzah (and allows him to become a "wall") since he is not aware that he is being used as a Mechitzah!
Q. Reese, Atlanta, Georgia
The Gemara actually concludes that one may use a person as a Mechitzah only if he does not know that he is being used as such.
As for your question, we discussed this in our Shi'ur, where it was suggested that unlike an animal (which will walk away simply because it sees food), people who are going somewhere or doing something specific, are unlike to just walk away for no reason (this is of course, before the age of cell-phones); and even if they do, the person who does know that they are being used as a Mechitzah, will normally have no problem in getting them to remain on some pretext or other.
be'Virchas Kol Tuv
Eliezer Chrysler.