I can't think of where or why Plato would have written this.
I'd suggest two things. One, conversation can itself be a form of play. Two, if the point is that you learn more about someone from the way she acts than from what she says/does, this is accurate. "Judge them by their fruits, eh? And a whole host of similar thoughts -- they honor me with their lips, but their hearts are far from me; the sheep had no idea they served Jesus in "the least of these."