Acting for Two: Candy Clark plays both Mary-Lou and the wife Thomas left behind. This isn't obvious because the latter is a Rubber-Forehead Alien who has no dialogue. (She also played Thomas himself in one scene, with a hat pulled over her face, when David Bowie was ill.)
Now, Ziggy Stardust was an alien on Earth And he was doomed To fall to human vice, and leave his people blue Director knew! Nicholas Roeg must have known 'bout this Banned from space, this is a case Of casting your actor... based on his works!
Bad Export for You: The original US distributor cut 20 minutes from the film, rendering it even more confusing. They were restored in the 2011 re-release.
Reality Subtext: Bowie was addicted to cocaine and other illicit substances at this point in his life, so seeing him play a character who falls under the sway of substance abuse has this trope written all over it (indeed, the filmmakers were aware of this). Perhaps fittingly his albums Station to Station (1976, recorded at the lowest point of his addiction) and Low (1977, the first album of the "Berlin Trilogy" that unfolded as he gradually emerged from it) got their cover art from photos of him here. As well, the look and to a lesser extent personality of his Thin White Duke stage persona for the former album and tour was adapted from his work here.
Unintentional Period Piece: The story supposedly takes place over several decades, but the fashions, technology and virtually everything else remain pure 1970s. This isn't helped by the fact that We Are as Mayflies to an Alien Among Us hero who isn't physically aging, meaning that only the appearances of the supporting characters clue us into the passage of time. On top of that, just the fact that David Bowie plays an alien clearly dates it as in the decade of his Ziggy Stardust sci-fi glam phase (by the time the film was shot in 1975, he had already moved on from that persona and sound).
The novel was primarily set in Kentucky, but the movie used New Mexico to take advantage of a tax break for British filmmakers (this was the first British-produced film shot entirely in the U.S.). Its deserts also provided a useful backdrop for the scenes set on Newton's home planet.
Bowie originally was tapped to write the score and worked on it with Paul Buckmaster. For various reasons, it wasn't used and has never been formally released, though a backwards bass part on Low's "Subterraneans" was taken from what they worked on.
Roeg also thought about casting Michael Crichton as Newton. While that didn't work out, it did lead Roeg to start thinking about making the film a Non-Actor Vehicle.
James Coburn's name was kicked around as an early possibility to play Nathan Bryce, but he would've been way too expensive.