Suicide is Painless was the themesong for the TV show and the movie. The lyrics were sung in the film by Johnny Mandell. In the film, the song was son during the Painless Pole's "last supper".

Loudon Wainwright played Captain Spaulding (now where did he get that name from?? Say the secret word....)and he did sing a few songs, but not Suicide is Painless. One of those songs may have been sung on a hill, but I don't recall.

Incidently, Mike Altman was Robert Altman's son and he was 14 when he wrote the lyrics.   His father joked that he only earned $70,000 for directing the film but his son made $1,000,000 off the song rights!