Answer: Perhaps nothing! Honest. It is possible that this person -- difficult from the point of view that he is not social -- is not someone you should spend time trying to change.

The real issue you need to consider is whether the person's behavior actually interferes with the ability of the group to achieve desired outcomes. For example, does the person participate actively in group projects, contributing ideas and using good group processes? Does the person respond in a courteous manner to requests for information? Does the person complete his individual work and provide that output to his co-workers consistent with your expectations?

In other words, perhaps you have confused the notion that someone is "difficult" with someone who simply is not very sociable. I would focus less on changing this person's behavior -- assuming it is not disruptive to your work -- and exert more effort helping the rest of the staff to understand that not everyone is equally social. It would be important that the rest of the staff, including you, accept his difference without trying to remake him in everyone else's image.

If the person's behavior is in some way truly disruptive, then I would approach it the way we would any job related performance problem. In particular, I would conduct a counseling session, and engage in follow-up discussions to help address that issue. If in doubt about specific expectations with respect to resolving such issues in your own agency, you should contact your HR Director for additional guidance.