Lee Marvin drank real alcohol throughout the production, even though director Joshua Logan fought him about it. In most movies, the actors and actresses drink tea for whiskey and water for vodka. Marvin would only work if he got real liquor.
Jean Seberg's singing voice was dubbed by Anita Gordon. Clint Eastwood and Lee Marvin did their own singing. Marvin's recording of the song "Wanderin' Star" went to number one on the U.K. charts, earning him a gold record.
Following The Sound of Music (1965)'s lead, director Joshua Logan decided to shoot on-location. He commissioned a huge mining town in the middle of Oregon's Blue Mountains, which was painstakingly constructed over seven months. This caused this movie to run wildly overbudget before filming even began. The location caused logistical nightmares: cast and crew slept in tents on-location, constantly running low on filming supplies, food, and other amenities. The stars were taken to and from the location by helicopter.
Jean Seberg (Elizabeth) described Lee Marvin's (Ben Rumson's) singing as "like rain gurgling down a rusty pipe".
Alan Jay Lerner: Singing while standing on the log next to William O'Connell during "There's a Coach Coming In."