They will never change unless you do it manually in battles. My first play through of the game I never knew about it at all until the end of the game. If you just want a quick break down, BR gives counter chances and Unarmed damage, FA increases magic damage given and taken.


*Warning, more in depth mechanics below*
As a general rule 1 Brave is equal to 1% chance to trigger your counter ability. So 40 BR would be a 40% chance to counter, 70 BR (max you can recruit a generic with) would be a 70% chance to counter. 97% chance( max you can raise it to permanently) combined with a counter ability that avoids the attack gives you a very broken 3% chance to be hit at all. Also, for unarmed damage multiply by your BR as a percentage. So attacking unarmed with a character with 70 BR will do twice as much damage as a character with 35 BR. You can't raise Brave easily until Chapter 2 when Ramza learns a unique ability to do it.

Faith predominantly affects spells, generally [Caster FA as a %] * [Target FA as a %] * [Spell effect chance or damage]. So the more FA your casters have the more damage/healing/success rate they will have, same with FA of the target. Also, if they have above 89 FA they leave your party forever. No joke. FA is much harder to raise than BR.

Hope I didn't get into too much depth and ruin the mystique of the game for you. You will notice FFT fans generally like to delve fairly deeply into the nuances and mechanics of the game.