Figure out which camp the player falls into, and then USE it. There's always a first, of course, but until you meet one, I'd also suggest avoiding using those words. Reassurance that yes, you ARE taking the character's skill into account, but you'd still like the player to TRY, is sometimes needed for players who, quite rightly, don't want their suave characters to be done in by their own social ineptitude. So you have to make friends. Pay less attention to players who are being boring.
Interesting characters tend to generate interesting sub-plots.
Yes, it does. NPCs, NPCs, NPCs. This encourages people to stop thinking about characters in utilitarian terms. If the players come to accept this, then their own characters are more likely to stop acting like plot devices themselves. Get the PC's to belong to some organization which sets some standards of behavior.