First thing is tell players how you run your games. I do not allow illegal activities for player characters except under exceptional circumstances. I view rules as guidelines, not as set in concrete, and I tend to modify every game that I play or run. As Gamemaster, I run the game, and if a character starts acting in a manor that I do not regard as acceptable, he will get one warning, and then bad things start happening to him and the group as a whole. Rules lawyers will be shown the door very quickly.
If they have a D&D background, I ask them what their character alignment normally is. Evil types, especially chaotic evil types, as advised to look elsewhere.
I ask them if they are going to take good care of the NPCs that they may hire. I normally role up significant NPC hirelings, give then a background, use them to feed information to players, and keep track of what they do, as recurring characters for use in a campaign. If they start to use the NPCs as "security men" or cannon fodder, bad things will start to happen.
After that, anyone still left is asked what type of character are they thinking of playing, and I see if we have a functional group. A range of characters seems to work best. Then I have them role up characters and based on those, figure out what sort of adventures/quests to develop. I like to run more trade-based or quest-based games. I am not interested in setting up a mercenary outfit for their benefit or running a small war. The sort of things that I like to run are things like Leviathan, Chamax Plague/Horde, Research Station Gamma, and Twilight's Peak. I am not into Mercenary, Striker, or High Guard.
As an example why I ask what type of D&D characters they may have played or play, here is what a friend and I did to one teenage Dungeon Master at a local convention. We were handed pre-generated characters, basically all of which were some type of evil. I was given a chaotic evil magic-user to run, and my friend would up with a very strong fighter, also chaotic evil. The DM said that we had to run them as chaotic evil, so I and my buddy had a quiet discussion of how we would play that, and then proceeded. It was an overland adventure taking a couple of days to get to the target. My buddy and I volunteered for the first night watch. Once the rest of the party was asleep, I thoroughly reinforced their slumber with several sleep spells, and then we cut the other 5 throats, and split up all of their goodies. (Think: A Few Dollars More) Needless to say, the rest of the players were not amused. We just looked at them and asked if they had any idea as to how chaotic evil characters would act when a guaranteed rich haul is before them verses a highly uncertain reward in the future. I should note that at the time I was 30, my buddy was in high mid-20s, and we were both seminary students. We were not impressed with being told how we had to act. We left the DM and the rest of the group to sort things out.
