Relevant Stories

The characters are hired by the good King Lothar to rescue his son, the true heir to the throne of an ancient dynasty, in order to prevent the King's evil Brother from placing his cruel bastard son Mordin on the Throne and disrupting the Merciful God Pelandrinor's perfect balance of order and justice in the peaceful land (Whew, was that a mouthful).

The only problem is that the players are immoral anarchists who don't care about law, order, the heavenly ordered way, goodness, the laws of succession or anything of the like.

Yes, you can simply bribe the players with enough swag to get them on the adventure, but they aren't going to be on the Game Master's side.

The Game Master will be a hated parent trying to make them eat their vegetables.

Pay attention to the characters that your players design. If the group is a ragtag mix of all sorts of squabbling factions, give them a reason to be working together. But work on motivation. You don't have to give each character the same reason for going on the adventure. Luke Skywalker was going on the mission because he wanted adventure, because he wanted to be the hero, because he wanted to help the man who knew about his father. Obi-won Kenobi wanted to do the right thing. Han Solo wanted money so that he could pay off the bounty on his head, hopefully with a little left over for himself. Chewbacca was there to help his life-long friend. R2-D2 and C-3P0 were there because Luke owned them. Princess Leia was in the rebellion because she believed in an ideological cause, and later because the Empire had destroyed her home planet. No two characters shared the same exact motivation.

This is where backgrounds and similar become important. If the players haven't given you much to work with, it is entirely possible to run solo-adventures with each player ahead of time in order to establish some background information and get players some motivation.

Besides character motivation, you also have to speak the character's language. A heroic space pilot will likely give the refugees a ride off-planet for free, or at least a greatly reduced price of whatever they can pay; but a less savory space pirate will likely need to be promised a lot of money (at least more than he can get by selling the refugees into slavery or back to the authorities) in order to get him on that adventure. Look at the dominant motivation of the party and design the core motivation around that. If the party is really diverse always remember that wealth, power, and their own lives are the things that almost every player values and thus most characters tend to value these things as well.