The Me and Bobby MCGee singer, who died from a heroin overdose in 1970 at the age of 27, is the subject of a new film by Oscar-nominated director Amy Berg, titled Janis: Little Girl Blue.

The documentary, which is finally released in the U.S. on Friday (27Nov15) after eight years in the making, includes interviews with associates of the star, but Berg has now revealed the centrepiece of her film is a series of haunting notes from Joplin.

Berg says she waded through a treasure trove of "thousands" of letters from the singer to her family which reveal her most private thoughts. In the documentary, they are read by singer Cat Power as part of the film's narration.

The filmmaker tells Uncut magazine, "The themes of this documentary had to do with validation and (Joplin) trying to be proud of herself and to straddle a line between her childhood and adulthood - so those were the letters I used... It was a beautiful way to include Janis' voice in the film. I wanted to make her world feel relatively small so people could get a glimpse of what it was like to be her."

Berg also reveals the film explores the anguish of Joplin's childhood and the effects on her psyche of missing her school prom because she was not invited, adding, "Janis went from being this adorable, bouncing, sweet kid to a kind of angst-ridden teenager/adolescent. The transition was really, really difficult for her. Success and fame don't really erase the pain of a troubled childhood."