oh, speaking of Lord Vetinari, i just thought about our introduction to him as a major character in Guards! Guards! and his first interactions with Sam Vimes, and the Lord Vetinari shown in later books.
yes, Terry Pratchett was fleshing out his characters as the series progressed, but I really think, that while Vetinari will always be the same character introduced in G!G!, he became a better person, very subtly so, hidden beneath his dictator mantle, because of Sam Vimes.
And not, “better person” suddenly becomes a super good guy, but Vetinari I think becomes more aware of the little people, the everyday people, through the Watch, and also in dealing with Sam, and seeing Sam become lit with very tightly contained righteous fury at all the injustice in the world.
Sam might’ve been allowed peeks into the inner thoughts and mechanization of Vetinari’s mind, and boggled at how complex and dark it was, but I think the constant vigilance of Sam against anything despotic kept Vetinari slightly more human. Sam inadvertently, through his own very nature, kept Vetinari in check.