The movie will premiere at the Sundance Film Festival in Utah, and the rock singer can't wait for fans to hear her latest recordings.
She tells MTV News, "I've been talking about wanting to score forever, and this was my first opportunity to explore that... It's going to surprise my fans. It's not what you'd expect; the film is very dark, very introspective. There's not even a lot of dialogue, which is great, because to me it makes this beautiful, sad platform for music.
"There's a lot of blending of sounds, a lot of ominous tones. I play a lot of keyboard... There's a lot of low drones... There are a lot of moments where Dave and I just built walls of sounds out of blaring cellos and trombones and synthesizers and harp, all on top of each other.
"It's not, like, a soundtrack. It's an atmosphere."
And Lee is now hoping to reteam with Eggar, whose cello work can be heard in films like Anonymous and Beasts of the Southern Wild, for more film work: "Dave and I were already talking about taking a meeting for another film, and I definitely want to work with him again."