Usually any longer conversation with Zoltan leads to Crowning Moments Of Awesome.
Whenever people drink. Geralt's reactions to Dandelion's ramblings at The New Narakort are especially hilarious.
Geralt's response to the Professor's "intelectual" Hannibal Lecture.
Geralt: Shut the fuck up.
When Geralt talks with the Fisher King. The man speaks only in grunts, but Geralt is completely calm and asks detailed questions. The best part is when every response is a "mhm" until the witcher asks if the villagers are getting along with the vodyanoi. The Fisher King's response? A long "oooooooo".
And then, the witcher provokes him to say a few words with an insult. The result? The man acts very mildly insulted and showcases the language of a classy British gentleman.