What is the character?
A paladin might be religious, but she might not have ever really given much thought to metaphysical concerns. She might be a warrior, but she might just as easily be a frail and bookish sort. And she might come from privilege or poverty or anywhere in between. None of those aspects informs a paladin.
What makes a paladin is a code of honor and a dedication to measuring her worth in the number of lives she saves.
Traditionally, those features are hard-baked into the eponymous class, but in many cases the lines begin to blur. While pen-and-paper games can enforce behavior with a bit more effort, it's very easy to have a character of the paladin class in a game without the character actually being a paladin. To be a part of the archetype, the character has to be willing to give her life in defense of what she believes is right, to keep the codes of honor alive as best she can, to show all of her virtues in word, thought, and deed. It's an intensely difficult road, but it's one walked with a light heart.
What's the angle?
Adventuring parties run along a continuum, with a band of pure heroes on one extreme and a gang of barely restrained amoral thugs on the other. A paladin will be the first one on the block to volunteer just so she can try to steer others clear of murdering thuggery. That's assuming that she hasn't already found one injustice or another so reprehensible that she feels a personal crusade is necessary. The archetype lends itself to self-starting individuals, preferably those with the means and will to try to change the world for the better.
There are also hooks for miles implied by a code of honor, and while the idea has long been played with, there's nothing to say that you can't find a new twist -- or an old twist that works particularly well. Refuse to attack an opponent whose back is turned? Refuse to strike the first blow? Try to be charitable to your enemies? They might cause some occasional mechanical difficulties, but all of them are long-standing rules that paladins have had, and all provide for a great deal of potential storytelling potential.
What makes it interesting?
It's worth noting that the paladin has a long history of being potentially annoying. Depending on a paladin's connection to the rest of the group, she can come off as overbearing, preachy, and self-righteous to insane degrees. That's a surefire way to make the archetype not interesting, because no one will ever bother to learn about your character due to the difficulty in learning about characters no one wants to interact with.
When played right, the paladin leads not by words but by example, throwing herself in harm's way -- often for teammates who wouldn't do the same, sometimes for people who as much as say that they wouldn't. She tends to provide opportunities for adventure, a meaning, a purpose. Sometimes she gets to be the one who stands in front of monsters and tells her companions to run; sometimes she's just the one who goes rushing into the dungeon because she knows someone is in danger within.
And if it needs to be said, anyone who adheres to a strict moral code is occasionally going to lapse, voluntarily or otherwise. She's going to have to make hard choices, and there's a lot of conflict that can be found there. She's still human, and she may very well sometimes wish to do something that she ought to avoid -- and sometimes, circumstance or design will force her to do so.
What should I keep in mind?
If you're going to get into conflicts with other members of your roleplaying group -- and paladins do tend to wind up in the center of moral storms -- take a very gentle hand with the whole process. You don't want to be seen as the nanny. While your character might believe that the group is doing the wrong thing, there's a case to be made for her going along with it insofar as she wants to try to reform her companions via action.
That having been said, paladins can make truly exemplary villains. While I don't generally cite good examples in words, Order of the Stick does a fantastic job showing how someone can be honorable, dignified, and good in so many ways... and yet be a reprehensible villain just the same. (Start here if you've never read the storyline in question, and come back to blame me for ruining your weekend.)
Above all else, roleplaying a paladin as an archetype is a challenge. You're going to have to either alter your playstyle or be a hypocrite. You're going to be constantly up against threats that you have voluntarily handicapped yourself against, and in all likelihood your character knows her life will end in battle against something horrible, possibly serving as nothing more than a minor inconvenience. It's a difficult road, fraught with peril.
If you have to ask why anyone would ever do that to herself, you might not be the target audience.
That's our fourth archetype, and it's probably the one closest to my heart, albeit one I could spend a lot more time talking about at length. (Like I'm known for my brevity to begin with.) Feel free to leave comments, questions, or feedback in the comment field, or send them along to firstname.lastname@example.org. Next week, on the eve of a new year, I'm going to be taking a swing at something new as well with a look at something more specific than the general advice I've been working with thus far.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.