What really comes across when you watch this Bowie documentary (if you watch it; it's really only of interest to hardcore fans) is, besides that Bowie is obviously losing his mind, that Bowie's whole shtick has less to do, at this point, with the music than with the fact of acting, the concert as drama. He doesn't go on about his music, but he has a lot to say about his creation & destruction of personas, his various costumes.
That's his whole shtick at this point in his career (1975). He's tried on & tossed off several popular genres (the weird folk-pop of Space Oddity, the "hard rock" of The Man Who Sold the World), but more importantly, the albums that are distinctively his sound (Ziggy Stardust in particular) emphasize this self-distancing. It's not just that Ziggy Stardust and Aladdin Sane are personae, it's that they're temporary personae. E.g. in the album Aladdin Sane, you've got the title track, which obviously isn't sung by Aladdin himself because it talks about him in the 3rd person. And then you've got "Cracked Actor," from which the documentary draws its title, sung from the perspective of a washed-out celeb (not Bowie himself), and then a preening parody of Mick Jagger in "Let's Spend the Night Together."
So where's the real element, where's the actor behind the mask? Because he can't just be a non-person who acts in interesting ways, that's not permissible for a modern celebrity. The press doesn't know what to make of him; the first clip in the documentary is about just that. They're confused, because Bowie doesn't know either--he's spent a lot of time developing the parts for his show, but not a lot developing his own part. This is the same year that Young Americans comes together, which is a big shift--instead of a plethora of parts, it's one big one, Bowie's "plastic soul" (a thankfully short-lived sound for him) and the persona that goes with it. The part makes him good money--album sells like crazy &c.--but he's not satisfied with it. (outtake from this album titled: "Who Can I Be Now?")
The old law of criticism is that one shouldn't conflate the artist's biography with the work. Which works to a certain extent with, say, a novel or a poem, but with a rock song--especially a song by Bowie--the divisions don't hold in place. The essence of modern pop music is that the art is inseparable from artist: "Super Bass" means nothing minus Minaj, "Imagine" means nothing minus Lennon. So one can't resist pointing out that, as Station to Station comes out & Bowie takes on the crypto-fascist "Thin White Duke" persona, he's coked out of his mind & not eating & his marriage is falling apart.
Part of the interest in following Bowie's career (which, after the early 80s, trailing off pretty fast) is seeing the way he navigates the artist/art division & resists the intrusion of the press (which he can't do completely simply because modern pop stars are required to have their lives on display to a certain extent). It's an issue particularly relevant given the rising idea of a "personal brand" and developing a fixed & unified online presence which is continuous with one's career. (You are your own capital, &c.) Bowie overcomes this through an excess of personality: if he's got to have an identity, he'd might as well have three, or four, or a different one for every album. The press's desire to make a "personality" out of an artist, like the corporate world's desire to make a "brand" out of a person, is of course a matter of control; David Bowie is exceptional for the way he resists this control.