A by that time controverse character and plot he was granted with was his role in St. Elsewhere. Little was known about Aids by then therefore prejudice were many. Though his character Bobby Caldwell was contrary to popular belief none of the risk groups, they still let him die from this immun deficency.
Mark Harmon has always been one to choose his roles in accordance with his likings for the character. Maybe that's the reason why you don't doubt him completely filling that role no matter if it was someone by the likes of serial killer Ted Bundy, a cop named Dick Cobb, the president of the United States in Chasing Liberty or a NCIS Special Agent called Leroy Jethro Gibbs.
All those characters have one thing in common: They live on the screen from the minute camera starts to the minute the picture fades off, breathed to life by the tremendous talent of Mark Harmon, who is one of the few able to speak whole monologues without saying a word. But I guess, I don't have to tell you that, probably the reason you came here in the first place. His abilities to get the different roles across brought him some award nominations even in his early years and some winnings as well.