He is best known as a cast member on NBC 's `` Saturday Night Live '' from 1998 -- 2006 and for his role as Dr. Leo Spaceman on NBC 's comedy series `` 30 Rock '' .
He is best known as a cast member on NBC 's `` Saturday Night Live '' from 1998 until 2006 and for his role as Dr. Leo Spaceman on NBC 's Emmy Award-winning comedy series `` 30 Rock '' .