In previous and subsequent episodes, William Christopher played Private Lester Hummel; here he's Corporal Haynes.
Sergeant Carter is assigned to work at headquarters for only 2 weeks on the same base. In other episodes, his quarters are in the duty hut for Company B. Since he is on temporary duty on the same base, he would remain in those quarters, and thus would return each day to the company area and could easily keep tabs on how Corporal Boyle is doing and how the platoon is doing.