One of the best parts of my Robert Altman voyage has been discovering how great of an actor Elliot Gould is. One of the 70s most underrated. His work in "The Long Goodbye", "California Split", and "M*A*S*H" is, in my opinion, sensational.
Not an Altman film but I saw Bob & Carol & Ted & Alice recently and he was great in it. Love this guy.
Also...one of the more interesting actors from that era, too. He's got a helluva lot of stories!
http://www.independent.co.uk/news/people/profiles/elliott-gould-i-didnt-have-a-drug-problem-i-had-a-problem-with-reality-7956788.html