“Rhys already knew about the Secret, right?” said Vimes.
“No, sir, not as such. But I think he had some reason to suspect there was something down there.”
“Then why didn’t he just go and look?”
“Dwarfs digging around in Koom Valley? The trolls would, er, go postal, sir.”
“But not if the dwarfs were merely investigating why a copper from Ankh-Morpork was chasing some fleeing criminals into the caves, right? Not if the copper was good ol’ Sam Vimes, who, everyone knows, is as straight as an arrow even if he’s not the sharpest knife in the drawer. You can’t bribe Sam Vimes, but why bother when you can pull the wool over his eyes?”
“Look, sir, I know how you must feel, but… well, there’s your little boy there, playing in Koom Valley, with trolls and dwarfs all ‘round, and they’re not fighting. Right? I didn’t lie, I just… liaised a little. Wasn’t it worth it, sir? Hah, you really worried them when you wen tot the wizards! Shine hadn’t left the city! Rhys had to fly him in by night! All they really did was follow your lead. The only person who fooled you was me, and it turns out I wasn’t very good at it. They needed you, sir. Look around and say it wasn’t worth it…”

– the importance of Sam Vimes |
Terry Pratchett, Thud!