Eh, not to spoil anything, but I'm not particularly proud of my performance in the Dark Sun campaign so far. Playing a Lawful Good alignment has been somewhat tricky two games in.
A literal read of Lawful Good in Dark Sun is probably feasible. Somehow. But I wouldn't personally volunteer to try it. Moral compromise in the face of terrible odds is kind of central to the themes of the setting, at least as I understand it. In keeping with my habit of telling strangers on the internet how to play and run their Cops and Robbers games, however, maybe you can spin it one of these ways:
The Fanatic: Your belief system is strictly defined and those who don't follow its tenets to the letter are not only wrong, they're
dangerous. You're intolerant, judgmental, and intractable to the point of insanity. On the up side, you have clear responses to any problem, can be utterly relied upon to obey your word, and anybody who falls under the protection and accepted boundaries of your faith will probably see you as a paragon of virtue - at least until the point they fall out of your favor. In the short game, your fellow PCs will hate you and will try to manipulate your beliefs to their own ends - assuming they don't just bury you neck deep in the sand and leave you behind. In the long game, you could explore your PC coming to realize that his ideals aren't applicable in the "real" world, which could be a crisis of faith or just a mellowing/moderation of his beliefs. You could also go for the other extreme and have him follow his righteousness to its inevitable end in self-destruction, which can be cathartic in its own way - provided it doesn't ruin the fun of the game, of course.
The Hypocrite: Embrace the contradictions in your PC's words and his deeds, pointing them out Out of Character while letting him be oblivious In Character. It could be that he never had to really
test his code of ethics before and he's constantly coming up short without realizing it. Or perhaps he thinks he's an exception to the rules. Or he's on the drugs. Whatever the case, the delusion helps him function and without it maybe he'd fall to pieces. Truthfully, this is how a lot of real people function; just think of all those conservative Republican congressmen who were caught getting handjobs at Quik-E-Mart. In other words, be David Vitter! In the short game, you can have fun casting the first stone from your glass house, pointing out others' flaws while overlooking your own (even when they point them out to you). In the long game, you could eventually have a crisis of conscience and a melodramatic meltdown which leads to your redemption OR the adoption of a new delusion. Consider how many of those same congressmen later got "cured" of their homosexuality or whoring by the power of Jesus.
Ye Of Little Faith: You espouse a philosophy or a religion, but deep down you don't really believe in it anymore. Maybe you just say this shit because you feel like its expected of you or because its something to cling to in the face of Ultimate Oblivion, but unlike the Hypocrite you KNOW that it means nothing. This is a good excuse to plant some backstory plot hook about how you used to be faithful up until That One Day when some traumatic incident took your treasured belief and curbstomped it. In the short game, you can be a gimdark reflection of the world's state and the voice of doubt in the party, sulking and getting drunk when you feel especially broken. In the long game, you can work on either restoring your lost faith or replacing it with something new and worthier. I suppose you could alternatively wallow in despair and eventually commit suicide, but I don't know that it'd be as much fun to play.
All of these are contingent on your willingness to display overt weaknesses in your PC and possibly incur the annoyance of your fellow players' characters (if not the players themselves). Depending how you present it and what understanding you can reach with the group, however, it could be a nice change.
For what its worth, I ALSO like playing Good Guys, but I've found that they're much more interesting when you let them be human, fallible, and have some issues to work through. Starting out as an inflexible holier-than-thou type can be hard to play in ANY game. On the flip side, starting with a decent person who is at a low or difficult place and
then working your way to something resembling heroism or redemption can be quite fulfilling. Even if they never quite get there, at least you've got room to go
somewhere.