The first guest
Shokyaku should be someone who knows Tea best, because he or she：
- does all talking for Guests
asks questions of and interacts with Teishu
- can help other Guests, but should not teach.
writes reply to Teishu's invitation.
contacts other Guests, who may also write replies. This is the time and place to warn the Teishu if you (or any of the Guests) have any allergies or if you do not drink sake. It is very distressing for the Teishu to come unprepared to a guest who refuses sake, without having thought of something alternative to serve.
Actually goes to Teishu's house the day before for zenrei, to be sure of route, that there is no change (fire/ disaster, funeral, etc)
Aisatsu in genkan, never comes in
During a Chaji Shokyaku sits nearest the Tokonoma during the Kaiseki/First Seki. Where he sits during the serving of tea depends on the room. If it is a Koma he sits closest to the Tokonoma. If it is a Hiroma he sits closest to the host. This is mainly significant in a hiroma with Gezadoko, because in other configuration he always sits the same place.