Some games I take the order in which people signed up and apply a nice pattern to give out roles, but order the names alphabetically so nobody notices. Some games I choose very specifically who I want in what role. I'm also often accommodating for special requests. If somebody really wants to be a certain role, I don't mind letting them have a shot at it. After all, it's my job to make sure everybody has fun.
With that in mind, when I do hand-pick special roles, it's usually because I think it will maximize the enjoyment of all. Only the most crafty and devious of bad guys, the most skilled intelligence agents, etc. I try to make the game as challenging as possible for both sides. Other times I try to force players into taking a different role than they're used to. Maybe somebody that usually is very active and takes a lot of risks I'll try to give them a role where they will want to minimize their risk and lay low. Maybe somebody that's relatively new to the game looks like they're ready for a more interesting position. Maybe I notice that somebody hasn't had a special role for several games. Stuff like that. It often boils down to gut feelings as I attempt to set up an interesting game that will make people want to sign up in droves the next time I host. At 17 players, almost 18 if BTK had contacted me three hours earlier, I'm fairly confident that my methods have been overall better than most.
Obviously I won't be revealing how I selected the roles this game, I might have used any method. I will say that a fair portion of it was random though, and since some of the roles in the game are entirely new, it's pointless to speculate on how I might have selected things. It's also been scientifically proven that the average ASW forum-goer doesn't think the same as me, so just don't bother trying to get inside my head.