Vetinari, is an assassin and a tyrant (although often benevolent in his tyranny), his disguise is that of bureaucrat, and everyone knows it. Having him speak gently to you should be terrifying, if he agrees with something you said alarms bells should be ringing. He isn’t **** Mr Bean!
To create the right mix of sinister, intelligence, ruthless yet caring for the society he has created, it has to be a Daniel Day-Lewis!
It has to be someone you could imagine moving efficiently down a dark passage full of booby traps, knowing exactly where to step, where to pivot, spin or duck, and not end it with a prat fall.