Actually, "React as you imagine pigs reacting." according to this self-published source. According to another source from a decade ago by the very same author and published by Random House, a somewhat longer instruction:

> The guards have to know that every guard has to be what we call a "tough guard" [...] we need you to act a certain way. For the time being, we need you to play the role of "tough guard". We need you to react as you imagine the pigs would. We're trying to set up the stereotype guard – your individual style has been a little too soft.

Not "real prison guards", but "pigs", which latter carried in the 1970s a whole bunch of connotations that the former does not. As does "stereotype guard".