While in Vergen (on Iorveth's path), you have to play dice against a dwarf to win a quest-related item. After winning twice (gaining a sword and the quest-item), he says "I'm not playing anymore!" in a tone that makes it sound like he's on the verge of tears.
From the DLC, there's a quest at Vergen called "A Sackful of Fluff", where a man named Elthon contracts Geralt to collect Harpy Feathers for a client. Then you see just what "his client" wanted the feathers for. The deadpan conversation afterward is just icing on the cake.
For bonus points, you get Elthon's Trophy from completing that quest. All of Geralt's cynical comments and philosophical insights become a little surreal when he's got the Trophy stuck on his nose.
A combined Funny and Awesome moment - when first meeting with the Scoi'atel, Geralt demonstrates his prowess by mentioning that there are 4 elves hiding in a tree, and one of them is either on Fisstech or has a cold due to a wheeze. The expression on the leader's face is priceless.
In Chapter 2 of Roche's path, if you opt to get information out of the Visionary by "converting" to his religion. To do so, you have to imbibe a potion of his own make in front of a shrine and have a special vision. Said vision involves giant phallic "mushrooms" appearing in the forest, alongside an enormous chicken. Made all the more better in that reporting this back to the Visionary is exactly what he wanted to hear!
Once Roche and Iorveth first meet in the forests on Flotsam, Iorveth greets Roche with a long-winded, scornful speech in which he lists several of Roche's accomplishments, titles and says how he has spent the past few years setting up traps and devising plans as he eagerly awaited for the day when Roche would wander into his forest. Unimpressed, Roche bluntly responds by saying:
Vernon Roche: Iorveth - a regular son of a whore!
One in the beginning of chapter 2 (Ivoreth's path) and related to the main plot, the sorceress Phillipa needs an artifact with ungodly amount of power. She suggested one of the legendary Twenty Rings of Power, leading to Ivoreth quoting from the famous ring verse "...and One to bring them all and in the darkness bind them". Then Geralt envisions himself running barefoot up a volcano.
Another one in Vergen, when Geralt talked to a vendor in the market place:
Spark: Uh, so what does a witcher do, anyway? Geralt: We solve problems. Spark: Oh, I have a problem with my boyfriend. Geralt: Did you find his entrails sprawled on the porch and his head a few paces away? Because those are the types of problem we solve.
Another one from Vergen, when Philippa gives Saskia the antidote to the poison, finishing with a kiss through a Rose of Remembrance petal, we hear the town Elder, Cecil, say "Now that's my favorite kind of magic - lesbomancy."
As Roche starts his interrogation of shackled Geralt, he offers his hand to shake. The "very funny" dialogue option leads to...
The interrogation gets even funnier if you prove to be uncooperative.
Geralt: What do you want from me, Roche? I already told you it wasn't me. On top of that, I don't know who's behind it, and frankly, I don't really give a damn. Could've been anyone as I see it. King Demavend, even.
Geralt's exchanges with Roche seem to contain plenty of amusing snarking no matter the situation. From the prologue:
(Geralt and company are being harassed by a giant dragon.)
Roche: Is it true you witchers don't hunt dragons?
Roche: This one doesn't seem to know that.
Geralt: You want me to go over and tell him?
Talking to the noble and priest in the prologue of Assassins of Kings, Geralt can sock the noble in the stomach. Roche quips that the priest living and the noble getting slapped is win/win.
Upon reaching Flotsam in Assassins of Kings, go talk to Triss in the tavern. Asking her how she's been yields a Deadpan Snarker list of what it's been like stuck on the ship with the rest of the crew.
Triss: I can burp the Nilfgaardian emperor's title without reaching for a beer and I've discovered that Shorty's name has nothing to do with his manhood.
When Philippa Eilhart mentions that the customary title for her apprentice Cynthia is "Leashed Sorceress", Geralt gives a hilariously deadpan response: "Do you also use a muzzle?" Philippa gets offended and explains the completely innocuous background behind the term. Later you end up walking on the two in the middle of a BDSM spanking session.
Roche gets a small one that doubles as a Crowning Moment of Awesome near the end of Chapter 2 on his path. As he and Geralt charge through Vergen, killing every Kaedweni soldier in sight, he runs across a bridge...which falls out under him. Roche curses as he tumbles, rolls as he hits the ground, and without missing a beat, punches a Kaedweni soldier in the face as he gets up.
Upon first arriving in Vergen and meeting Yarpen Zigrin, the conversation includes this:
Yarpen: Well, Saskia went with prince Penis ... uhh, rather ... Stennis to negotiate with Henselt.
After succeeding to find a succubuses lair, who has allegedly been killing the men of Vergen, Dandelion faces a dilemma: