Let's put it this way, I've always deeply empathized with "B.A." from "Knights of the Dinner Table".
It's not so much that the players aren't serious - about certain things most of them tend to be very serious. Sure, in every table you have one goof that doesn't care if his character lives or dies and is just trying to be silly, but about survival and maximizing the chances of hitting a monster in a combat, the majority of players are very serious. If you let them, they'll argue tactics in a fight for 10 minutes. And regarding RP in as much as they think it will help them advance their character, they are very serious.
But about staying in character or creating some sort of emotionally meaningful experience or great story, they pretty much could care less. It happens occasionally, but mostly by accident.
So mostly I run a beer and pretzels game that people enjoy, even if it doesn't necessarily go where I would have wanted it to go.