The CIA Agent who questions John Barnett at the end of the episode is actually played by William B. Davis, otherwise known as the Cigarette-Smoking Man.

Mulder: He gunned him down just for spite. This is a man with a wife and two small children and you, you shot him without hesitation, without conscience, without an ounce of humanity. Which is why you should die like an animal, you son of a bitch!

Scully: What are you going to do?
Mulder: I know what I'm not going to do. I'm not going to hang around and wait for Barnett to send me another valentine.

In the scene where after Mulder gets the death certificate from the fax machine and walked toward the door, the microphone handle can be clearly seen in the top right corner.