Know thy Characters

They come rushing out of the jungle like a bloody wave. They look something like undead inside out chihuahuas with bigger teeth. There's hundreds of them, with saliva spilling out between their needle-sharp gnashing teeth. What do you do?
[insert gales of laughter here for 5 minutes - 10 minutes - 15 minutes]
Seriously, there's thousands of tiny teeth about to rip you slowly into bit-sized morsels.
Chase: (gasps for breath) I pick up my Thompson submachine gun.
Margot: (through clenched teeth) I break out my double-barrel shotgun.
Terence: (holds his aching stomach) I draw my Mauser pistol and click it over to full-auto.
Scott: (stiffles a coughing fit) I whip out my twin .45 automatics, the ones with the extended clips.
Anzo: (clears his throat) I kick open the box to the Lewis Gun and shoulder it.
The things leap at you...
Rat-atat-tat, Boom-Boom, Pow:Pow:Pow:Pow, Blam.Blam., Braaaaaaap.

Thus was born the "Wall of Lead". Not a single undead-killer-chihuahua so much as nicked one of the characters with their nasty tiny sharp pointy teeth. There was just red spray and jungle mulch. And worse, none of the characters, or Players, was in any way terrified. They just blew the smoke from the barrels of their arsenal, reloaded and went to sleep for the night (the characters, not the Players). They did leave a guard. (one freakin' guard)

My first reaction was anger. How can they not be frightened? I think inside out chihuahuas are really disgusting. The Players should have at least been disgusted. I was furious. Furious! Years later, after faaaaaar to many reminders of this incident, I figured out the problems:
1. Chihuahuas aren't scary. Ugly, maybe, but not at all frightening. Bad choice for my description.
2. Hundreds of not-scary little monsters doesn't suddenly make them frightening. It makes them hilarious.
3. I shouldn't have been angry. I should have been as amused as the Players were. It was an awesome display of firepower.
4. I didn't think about the CHARACTERS. I'd spent several game sessions making them afraid of everything that moved. Unearthly horrors had plagued them for adventure after adventure and their sanity was slipping. They were more than a little unstable and were making up for it with firepower - firepower they planned to use on much more dangerous adversaries later in the adventure.

The Wall of Lead was not an over-reaction. It was a response I should have expected. They'd acquired every piece of their armory during the course of play and I should have known they'd bust it out at the first sign of trouble. After all, I'd pushed them to that level of paranoia. Heck, they didn't even break out the grenades or dynamite.

These days, I keep copies of every Player's character sheets, so that I know what their characters brings to the party and I can create encounters that fit their experience. I've also learned to applaud the characters when they create a novel situation. And I've come up with BIG scary monsters. Really. They're VERY scary.