You can’t have your cake and eat it, too, in life,” Edith “Big Edie” Bouvier Beale quips to her daughter in Albert and David Maysles’ documentary “Grey Gardens.” But in portraying Beale in the HBO narrative adaptation of the film, Jessica Lange is defying her character’s words.

“It’s one of those parts that doesn’t come along very often and that I feel so blessed to have a chance to do,” Lange says. “It’s also one of those parts where I really fell in love with her. She’s such a unique character, and the fact that we had that documentary. It was like a treasure-trove.”

Lange ages 20 years in both directions to play “Big Edie,” the aunt of Jackie Kennedy who spent her twilight years living in impoverished isolation with her daughter, played by Drew Barrymore in the mini. While the documentary focused only on the Beales’ later years, HBO’s adaptation also traces the path that led these women from a life of wealth and privilege to one of poverty and eccentricity.

“The idea of taking this woman who has become an iconic figure and to provide an imagining of who she was 40 years earlier — that was what made the work really exciting to do,” Lange says. “But the possibility for failure was great. One of the reasons it worked is the connection that Drew and I had.”

With both thesps eligible in the lead actress category, Lange (no stranger to awards — she’s won two Oscars) shoots down any suggestion there might be any rivalry come Emmy night.

“It’s too bad there has to be a singular award,” she says. “They were two great roles, and it really is a two-handed piece. One can’t exist without the other. They are two halves of the same equation.”