The Senior Deacon is primarily a ritualist, with significant parts in all three degrees. He guides the candidate through the first section of each degrees, and is the primary person to deliver the second sections of the EA and FC, and a sizable part in the second section of the Master's. In lodge metings, he's also responsible for greeting and introducing visitors to the Lodge, attends the ballot box during voting, atends to the altar and its arrangements. He and the junior deacon work closely with the Master and the Senior Warden on miscellaneus errands and tasks as may be needed. Additional duties can vary, but he's usually fairly active in the lodge educational program as well.
Got that second section of the Fellowcraft memorized yet? December ain't far off ;-)
(I'll be moving up to the SD slot this December as well ;-) )