It is clinger laying in the coffin. He wants to kill himself and the others set up an elaborate scheme to make him change his mind. The song plays with the lyrics then. It is called Suicide is Painless.
I beg to differ... It was the dentist "Painless" who was in the coffin. He wanted to commit suicide because he lost the "lead from his pencil" so he took the "Black Capsule." The black capsule was not real.
The nurse, Lt. Dish, put the lead back in Painless' pencil and all was right in the world again.
I always liked the irony that the song sung while Painless tried to commit suicide is titled Suicide is Painless