Depth of Character

Skull-butting Hitler might have been momentarily satisfying for Dr. Jones,
but the long-term benefits would be questionable.

Vanir over at Critical Hits has a great post about playing PCs with a sense of depth. He uses the example of a character whose parents were killed by orcs and how that character might behave if faced with orcs. In a nutshell, it comes down to finding avenues to express a character's backstory in ways other than.

I LOVE THIS! MUST HELP HOWEVER I CAN AND HUMP LEG! 

or

I HATE THIS! MUST PUNCH AND KILL THEN HUMP CORPSE! 

As I was reading the article, I thought of existing fictional characters with similar backgrounds--ones that instill a strong opinion on a subject. Take Malcolm Reynolds from Firefly. The dude hates the Alliance. They killed his whole army or something. While he is much more likely to jump at the opportunity to pull a fast one on Alliance interests, he is also very discerning about how he does it. He doesn't charge blindly shooting every time he sees an Alliance soldier.

Similarly, Indiana Jones hates Nazis. He actually says, "I hate these guys" (as if we didn't know!) in Last Crusade. Yet, he frequently hitches rides on their Zeppelins, submarines and trucks... and even goes to one of their big book burning parties. While he does these things to counteract their Nazi plans, he does not just start shooting or whipping every time he sees a Nazi.

Major events in a character's backstory, whether in a game, a book or a movie represent only a part of that total character. These events can have a profound effect on the character's motivations and actions, but they should color those actions rather than dictating them. Just because the good cleric detects an evil aura around the grand vizier does not mean he/she should immediately bash his head in, especially when trying to get an audience with the king.