In the original Grey Gardens (1975), David Maysles asked Edith 'Little Edie' Bouvier Beale who she would want to portray her mother, Edith Bouvier Beale, if a movie based on Grey Gardens were made. She suggested Ethel Barrymore, Drew Barrymore's great-aunt, who died in 1959.
As pianist and music teacher George "Gould" Strong, Malcolm Gets's costume includes a diamond ring. It's actually the ring Edith Beale gave Strong in real life. Strong's relatives lent the ring to the film's production crew.
Little Edie's black bathing suit with yellow flowers is not a reproduction. It's a vintage piece from the same batch as the suit Miss Beale owned. It belongs to a fan of the original documentary, who lent to the film's production crew for the shoot.
At one point, Phelan Beale calls Gould Strong a "God-damned Ganymede." In Greek mythology, Ganymede was a Trojan prince whom Zeus kidnapped to be his lover. In other words, Phelan called Gould gay.
In interviews, Drew Barrymore said she stayed very close to Little Edie's character throughout filming, shutting herself off from friends and the outside world.