This article is from the ER FAQ, by Rose Cooper email@example.com with numerous contributions by others.
In "Dr. Carter, I Presume" , Carter has to wake up a
sleeping surgeon to ask about a patient`s treatment. The sleeping
doctor, whose name is Dr. Karubian and whose face we never see, was a
cameo role for none other than David Schwimmer, who plays Ross on
"Friends". NBC has confirmed Schwimmer`s appearance and has said that
no further appearances are planned. (The doctor was not played by
Chicago Hope`s Adam Arkin, as some have speculated.)