I know, but as I said, the n.p.c.s don't respond to the player's imagined responses so it doesn't solve the problem that he conversations are implausible and silly.

Either the player doesn't imagine a response, and the n.p.c.s are talking to a wall and act as if that be normal. Or the player does formulate a response, and Gordon Freeman is talking to walls, and acts as if that be normal.