Farmer: I can't take all the credit. My wife handles the business side of things. She has a good head on her shoulders!
PC: Hmm... Really?
During Act III while a demon ambush is going on, the recruits are panicking. Their commander...not so much:
Recruit: Demons! Captain, what should we do?
Captain Haile: Private, if you need to be reminded which end of the sword goes where, you haven't been paying attention!
Captain Haile: You there! Damotrius! Stop moping about! If this was your funeral I'm bloody well certain you'd have been told!
And when you're battling demons with him in the Blaze of Glory event at Rakkis Crossing:
Captain Haile: Those worms have come up from behind! Bloody adorable! They're trying to ambush us!
Also from Act III, when raising the last catapult from its moorings, your sole NPC aid comes from a grizzled lieutenant whose vinegar-laced "encouragement" straddles the line between hilarious and Crazy Awesome:
Lieutenant Clyfton: I'm sorry, catapult, I'm not strong enough to lift you on my own. Would you mind convincing my friend over there to lend a bloody hand?!
Made even funnier after you finally get the damn thing lifted, and he becomes amazingly sheepish:
Lieutenant Clyfton: Done! I...uh...promised Captain Haile I'd watch my temper. Do me a favor and don't tell him that I shouted at you a little, all right?
In Act I after rescuing Cain, you can stumble upon a conversation between a villager and a tired patron in New Tristram:
Villager: You know what would take everyone's mind off their troubles? One of Theodas's great comedies.
Barbarian: Yes. My people were the guardians of the Worldstone.
When the old soldier and his wife at Bastion's Keep in Act III aren't being heartwarming, they're usually being funny instead. In particular, after one event they reminisce together about the time she was kidnapped by barbarians: the soldier describes how he was beside himself until they returned her with an apology, and his wife comments that their leader still sends her a bundle of hides every year and is the nicest kidnapper she's met.
Kormac the Templar's truly over-the-top reactions upon encountering Elite Mooks. Made funnier when made against elites in Whimsyshire, the land of unicorns, cuddly bears, flowers and happy clouds.
BY ALL THAT IS HOLY! Do you see that enemy over there?!
In the Caldeum Bazaar in Act 2, there are two kids (a boy and a girl) running around with their pet rabbit. They eventually make their way to an Iron Wolf in the center of the map, where the man kills the rabbit with his sword, splattering blood all over the ground. The boy and the girl then run in opposite directions, crying.
Taking Lyndon through the bulk of Act 2 or 3. His endless complaints are rather amusing...helped greatly by his voice actor's excellent work.
Eirena: Do you ever feel like you are encased in webs and slowly suffocating in a lightless cave? Because I do not.
And there's this little gem of conversation in Adventure Mode between Tyrael and Lorath Nahr:
Tyrael: My stomach feels strange. Lorath Nahr: Did you forget to eat again? Tyrael: No. In fact, I decided to get the day's eating out of the way all at once. I kept at it until I couldn't take another bite. Lorath Nahr: Oh, it sounds like you ate too much then. Tyrael: Being a mortal is very complicated.
After players noticed this conversation happening almost every time they went into town, it was reduced, with the patch note, "Tyrael will discuss his digestion with less frequency."
Veteran Demon Hunter Greyscarr never shows up in the game proper, but the flavor text for some of the Demon Hunter's abilities imply that he has a wicked sense of humor.
Blood Vengeance: If it bleeds you can kill it. Kill this one slowly.
Archery: Feel the weight of the crossbow in your hand. Now... the wind, the distance, the target's speed. Good. Now try it with blood in your eyes and a demon at your throat.
Sharpshooter: Every creature has a vulnerability. Find it, and put an arrow in it.
Ballistics: How should I know who first invented these devices? Just make sure the correct end is pointed at the enemy and they'll do the rest.
In Reaper of Souls the player encourages Kormac to tell Eirena how he feels about her. Kormac does by asking her if she'd like to continue adventuring by his side after the current quest is over. Eirena excitedly tells him yes. And then she tells Kormac that she feels as close to him as she did to her sisters. Kormac is... less than thrilled when she says that.
Mystic: I see a village in flames, and an old friend will pay the ultimate price so that the truth may be revealed.
PC: (who has probably played through this part of the game several times by now) I know this already.
Mystic: Really? How do you know?
PC: Ah...never mind...
Some of the legendary items can earn a good laugh if you catch the reference they're making. For example, the Bottomless Potion of Kulle-Aid, which lets you break through the walls that wallers make. The best part is the flavor text:
When Imperius begrudgingly agrees to help you defeat Malthael in Reaper of Souls, he coldly informs you he won't thank you, even if you succeed. Most PC answer with an equally cold answer. The Wizard, on the other hand?
In Bastion Keep, a young couple can be heard talking in between quests. Eventually, the woman tells him that, when everything is over, they should start a family, with children and everything. The soldier doesn't know how fast he should go back to the battlefield. One would rather face The Legions of Hell than talk about children, after all.
Later on, he admits he was an idiot and he would gladly start a family. Then, when the siege is lifted and all the soldiers can finally get some rest, they have this conversation:
Soldier: It's all come down to a single fight, and I don't even get to be there! Wife: Good, you could use some sleep. Soldier: But I'm not even tired! Wife:(*seductive*) You could use some sleep... Soldier: Oh!
While seeking for the first sword piece in Act I, Lyndon lampshades the light coming out of cave entrances.
Player: This magic is familiar to me. This must be where the sword piece fell. Lyndon: Either that, or the goatmen have built one hell of a bonfire.
The Monk's journal entry upon encountering Covetous Shen:
That barrel is talking.
The Barbarian trash-talking the cowardly (former) mayor Holus of New Tristram in the beginning of Act III is hilarious.
Holus: Please escort me out of here! I will pay anything! Barbarian: Keep complaining, and I will escort you to the nearest catapult. The demons will know true fear when they see your fat carcass hurtling down on them.