Petula Clark "outed" herself and co-star Fred Astaire as smoking marijuana during the filming of 1968's Finian's Rainbow. "There was a lot of Flower Power going on," she told the BBC.
The movie, directed by pot-puffing Francis Ford Coppola, is set on an agricultural cooperative where Al Freeman Jr. attempts to develop a pre-mentholated tobacco. The plot has co-star Don Francks trying hard to get a hand-rolled cigarette to produce smoke, and ends with the whole cast blissfully doused in smoke.
The Canadian-born Franks—a jazz singer, poet and Native American—used to perform a song called Smokin’ Reefers. "A smoker of weed in his younger years, he was a fan of the plant. He gave up drinking when he was 21, using the First Nations term 'firewater' when referring to alcohol." Source.
I can't find any other reference to Astaire and marijuana, but the choreographer most closely associated with him, Hermes Pan, is described in a biography as offering both tobacco and marijuana cigarettes to guests at a 1949 dinner party at his home in Coldwater Canyon.
Astaire called Pan his "ideas man" and the two began their collaboration on "The Carioca" number for "Flying Down to Rio" (1933) (probably the most humorous dance duet ever). Pan also suggested Astaire dance with a hat rack in "Royal Wedding," and advised him how to do it. He continued to collaborate with Astaire right up until his last musical picture, Finian's Rainbow.
Pan was also close to VIP Diego Rivera, who may have turned him on to pot in Mexico, if Errol Flynn's account of his own experience with Rivera serves. Pan and Rivera met at the Palace Hotel in San Francisco in 1943, introduced by actress Paulette Goddard, who appeared in Charlie Chaplin's "The Great Dictator" and "Modern Times" as well as "The Women."
Shortly afterwards, Pan visited Rivera at his home in San Angel near Mexico City where Rivera asked Pan to pose for him dancing, so that he could work out techniques for depicting motion in his paintings.
Errol Flynn wrote in his autobiography My Wicked, Wicked Ways that he visited Rivera in 1935, introduced by another actress, Dolores Del Rio (who also appeared in "Flying Down to Rio"). Rivera offered Flynn marijuana, which he smoked, and afterwards he could hear the paintings singing.
"Pan found life in Hollywood even more superficial and insignificant after his return from San Angel," wrote his biographer John Franceschina. Maybe the two-week posing process included puffing something mind expanding and if so, he shared some with his friend Fred.