My games are "serious" in that the tone of most of the adventures is very grim, and I really only allow people to do things that are within the realm of real-world physics unless assisted by magic. I don't care if you got a 19 on your acrobatics check, unless someone cast the Jump spell on you or you have some sort of pole vault or rope, you're not going to make a leap across a 30ft gap on your own as a Halfling.

Now, the humor in my game comes from the characters interacting with each other through conversation or interaction with the world with these restraints in mind.

For instance, the party killed an assassin after interrogating him and getting nowhere. The next room in the dungeon had a pressure plate trap in place. The rogue decided to just throw the corpse onto the plate to trigger it, and then they just walked over him into the next room while the plate was still pressed.

Bard: "I dunno guys, we just tortured this guy and [Druid] just slit his throat, it seems kind of disrespectful."

Rogue: "Oh, come now, he kills people for a living, it's not like he has any morals."

Fighter: "Says the guy who is currently being hired by the queen to silence an uprising by any means necessary."

Rogue: "It's okay when we do it because we're the good guys."