Explaining intent in dialog

I sometimes wonder how, in prose fiction, description and dialog can support each other. One way you can use description to support dialog is to explain intent in prose. For example, this passage from Hogfather (Terry Pratchett):

"Is that all you're here for?" She [Susan] said. "A robbery?" He [Mr. Teatime] was dressed like an Assassin, after all, and there was always one way to annoy an Assassin. "Like a thief?"

I like the way this description supports the dialog. It's straightforward, one sentence, but it makes Susan's intent clear.

Incidentally, Mr. Teatime responds as she hoped. It's a fun exchange:

Teatime danced excitedly. "A thief? Me? I'm not a thief, madam. But if I were, I would be the kind that steals fire from the gods."

"We've already got fire."

"There must be an upgrade by now..."