Season: 5
Episodes: "Greene with Envy," "Sins of the Fathers," "Truth & Consequences," "The Peace of Wild Things," and "Humpty Dumpty."
Character: Dr. Gabriel Lawrence

He may have played a wacky doctor on Mash, but Alan Alda's five-episode stint as Dr. Gabriel Lawrence on ER. Alda's character comes to County General as the new attending physician and instantly clashes with Dr. Greene. As his misdiagnoses of patients and memory loss become a hindrance (as well as dangerous) in the ER, Kerry Weaver steps in to help the man who was her mentor many years ago.

He is ultimately diagnosed with Alzheimer's. This role earned Alda an Emmy nomination.

• The Biggest and Most Unforgettable Moments on ER
• More About ER