Kepler is certainly correct in saying that Randy is not inclined to believe a single word he says. The only way for Kepler to gain any credibility is for him to show up in person at this jail and utter the words face-to-face, which given all of the other things that he could be doing, for fun or profit or both, at this moment, gives a lot of weight to what he’s saying. It is implicit that if the Dentist wanted to lie, badly and baldly, to Randy, he could send his lawyers around to do it for him, or just send him a fucking telegram, for that matter. So either he’s telling the truth, or else he’s lying but it’s very important to him that Randy should believe in his lies. Randy cannot work out why on earth the Dentist should give a flying fuck whether Randy believes in his lies or not, which pushes him in the direction of thinking that maybe he really is telling the truth.