From "Manual of Muad'Dib" by the Princess Irulan

He's lying , the Baron thought. It must've been pretty bad .
"The Imperial lackey, this Kynes," the Baron said. "He was playing a double game, eh?"
"I'd stake my reputation on it, m'Lord."
His reputation!
"Have the man killed," the Baron said.
"M'Lord! Kynes is the Imperial Planetologist, His Majesty's own serv—"
"Make it look like an accident, then!"
"M'Lord, there were Sardaukar with our forces in the subjugation of this Fremen nest. They have Kynes in custody now."
"Get him away from them. Say I wish to question him."
"If they demur?"
"They will not if you handle it correctly."
Nefud swallowed. "Yes, m'Lord."
"The man must die," the Baron rumbled. "He tried to help my enemies."
Nefud shifted from one foot to the other.
"Well?"
"M'Lord, the Sardaukar have . . . two persons in custody who might be of interest to you. They've caught the Duke's Master of Assassins."
"Hawat? Thufir Hawat?"
"I've seen the captive myself, m'Lord. 'Tis Hawat."
"I'd not've believed it possible!"
"They say he was knocked out by a stunner, m'Lord. In the desert where he couldn't use his shield. He's virtually unharmed. If we can get our hands on him, he'll provide great sport."
"This is a Mentat you speak of," the Baron growled. "One doesn't waste a Mentat. Has he spoken? What does he say of his defeat? Could he know the extent of . . . but no."
"He has spoken only enough, m'Lord, to reveal his belief that the Lady Jessica was his betrayer."
"Ah-h-h-h-h."
The Baron sank back, thinking; then: "You're sure? It's the Lady Jessica who attracts his anger?"
"He said it in my presence, m'Lord."
"Let him think she's alive, then."
"But, m'Lord—"
"Be quiet. I wish Hawat treated kindly. He must be told nothing of the late Doctor Yueh, his true betrayer. Let it be said that Doctor Yueh died defending his Duke. In a way, this may even be true. We will, instead, feed his suspicions against the Lady Jessica."
"M'Lord, I don't—"
"The way to control and direct a Mentat, Nefud , is through his information. False information—false results."
"Yes, m'Lord, but . . . "
"Is Hawat hungry? Thirsty?"
"M'Lord, Hawat's still in the hands of the Sardaukar!"
"Yes. Indeed, yes. But the Sardaukar will be as anxious to get information from Hawat as I am. I've noticed a thing about our allies, Nefud . They're not very devious . . .politically. I do believe this is a deliberate thing; the Emperor wants it that way. Yes. I do believe it. You will remind the Sardaukar commander of my renown at obtaining information from reluctant subjects."
Nefud looked unhappy. "Yes, m'Lord."
"You will tell the Sardaukar commander that I wish to question both Hawat and this Kynes at the same time, playing one off against the other. He can understand that much, I think."
"Yes, m'Lord."
"And once we have them in our hands . . . " The Baron nodded.
"M'Lord, the Sardaukar will want an observer with you during any . . . questioning."
"I'm sure we can produce an emergency to draw off any unwanted observers, Nefud ."
"I understand, m'Lord. That's when Kynes can have his accident."
"Both Kynes and Hawat will have accidents then, Nefud . But only Kynes will have a real accident. It's Hawat I want. Yes. Ah, yes."
Nefud blinked, swallowed. He appeared about to ask a question, but remained silent.
"Hawat will be given both food and drink," the Baron said. "Treated with kindness, with sympathy. In his water you will administer the residual poison developed by the late Piter de Vries. And you will see that the antidote becomes a regular part of Hawat's diet from this point on . . . unless I say otherwise."
"The antidote, yes." Nefud shook his head. "But—"
"Don't be dense, Nefud . The Duke almost killed me with that poison-capsule tooth. The gas he exhaled into my presence deprived me of my most valuable Mentat, Piter. I need a replacement."
"Hawat?"
"Hawat."
"But—"
"You're going to say Hawat's completely loyal to the Atreides. True, but the Atreides are dead. We will woo him. He must be convinced he's not to blame for the Duke's demise. It was all the doing of that Bene Gesserit witch. He had an inferior master, one whose reason was clouded by emotion. Mentats admire the ability to calculate without emotion, Nefud . We will woo the formidable Thufir Hawat."
"Woo him. Yes, m'Lord."
"Hawat, unfortunately, had a master whose resources were poor, one who could not elevate a Mentat to the sublime peaks of reasoning that are a Mentat's right. Hawat will see a certain element of truth in this. The Duke couldn't afford the most efficient spies to provide his Mentat with the required information." The Baron stared at Nefud . "Let us never deceive ourselves, Nefud . The truth is a powerful weapon. We know how we overwhelmed the Atreides. Hawat knows, too. We did it with wealth."
"With wealth. Yes, m'Lord."
"We will woo Hawat," the Baron said. "We will hide him from the Sardaukar. And we will hold in reserve . . . the withdrawal of the antidote for the poison. There's no way of removing the residual poison. And, Nefud , Hawat need never suspect. The antidote will not betray itself to a poison snooper. Hawat can scan his food as he pleases and detect no trace of poison."
Nefud 's eyes opened wide with understanding.
"The absence of a thing," the Baron said, "this can be as deadly as the presence . The absence of air, eh? The absence of water? The absence of anything else we're addicted to." The Baron nodded. "You understand me, Nefud ?"
Nefud swallowed. "Yes, m'Lord."
Then get busy. Find the Sardaukar commander and set things in motion."
"At once, m'Lord." Nefud bowed, turned, and hurried away.