Crank up the Tension
Although talking to someone can work (and it is certainly an adult way to handle behaviorally issuesthings), often times people "are what they are" (as you've seen).
Some alternate methods I've used include:
- Ignore the behavior and move on. Without attention, the behavior might go away on it's own.
- Sidle up to an attentive player and start (game) talking directly to them. Soon everyone will start listening in intently.
- Give a clue of impending danger. An ominous sound nearby, a flock of birds taking flight, a fleeing NPC, a knock at the door (knocking on the table to simulate), etc.
- Start an action scene (combat or some other sort of peril)
- Play at a faster pace. Move through actions (and results) quickly, moving on to ready players if someone is not ready (no need to actually skip them though: you can come back to them when they're ready)
I've found that cranking up the tension (to the point that the players are sweating bullets and actually worried about what's going to happen next) focuses players and makes most behavioral issues a non-concern. Players tend to become more serious when put in peril.
And it doesn't even have to be constant combat, just a general foreboding. Example:
GM: The NPC notices your laughter and stares at you funny.
Player: (still laughing)
GM: The NPC whispers something to the person next to him
Player: (nervously laughing)
GM: That person then glances at you, and quickly leaves the room. Make an Insight check.
Player: (stops laughing)
Some of my best sessions involved foreshadowing. And (unlike dirty surprises) players tend to be much more cool about impending disasters if they've been given signs (even if there's nothing they can do about them).