I can primarily speak as a PC, here's my $0.02; hope it helps!
When I created my character, I leaned heavily towards the min/max style of creation--standard elad rogue, but I wanted hand crossbows.
From there, I took the attributes and started to think about how he would interact--as wisdom was my "dump stat" (much to the detriment of what makes a rogue a good scout...), I figured he'd be pretty normal, but not too observant in the nuances of conversation or common sense.
In addition, I'm fairly decent at mimicking stereotypical voices, while in character, I spoke with a faux-Russian accent.
Combining these, I started to reference the minotaur in our group as "big cow that talks".
----
I read somewhere that one way to fully flesh out a character is to imagine you're sitting down with him/her to do a one-on-one interview. I sort of second SeveredHead's advice about not relying heavily on back-stories, but you could ask "yourself" questions like:
* what they like?
* what makes them tick?
* what would they do if they entered a crowded bar?
* what would they do if a fight broke out in said bar?
* what would they do if they encountered a dragon (benevolent, neutral or evil, doesn't matter)?
* what question/topic would you (as the interviewer) ask that would make their character jump out of their chair, grab you by the lapels, and threaten you with significant violence?
Lastly, let them mirror characters they've read in other books. Are they Rincewind, exasperated and wandering through the land? Are they Japheth from Cordell's FR series--an easily-shaken individual with a good heart, but addicted to Traveler's Dust? Are they a general from any war era? Do they think like a Mentat ("Dune"), or would they rather be participating in Legolas' and Gimli's "Who can kill the most..." contest?
----
From a DM's perspective, I think the more YOU get into the NPC's personalities, the more comfortable everyone else would be (seconding Trit here). Maybe there's a form of "stage fright" happening? I acted out one PC in Harkenwold as a "drunker" Jack Sparrow, who kept falling out of his chair, and wondering where his bottle of mead is.
The other thing, like Quickleaf and Radiating Gnome said, could be that they are just battle-maniacs who want to roll fast-and-furious, and cleave through the legions of whatever you throw at them.
Hope this helps!
-CastX7