When Hannibal first meets Clarice, he talks about how he cannibalized a census taker:
A census taker once tried to test me. I ate his liver with some fava beans and a nice chianti.
He follows that line up by hissing at Clarice.
I'm confused what the meaning behind the hiss is. One of the defining characteristics of Hannibal is that he's obsessively polite and abhors impoliteness in others, and hissing directly at your conversation partner during your first meeting feels exceptionally rude. In fact, it sounds like something Miggs would do.
I've read a lot about the intricacies of Anthony Hopkins' acting in this film, is there any in-character reason for the hiss?