I used to spend a lot of time doing behind-the-scenes documentaries on feature films. Since I was working mostly for low-budget independents, the BTS stuff often dropped off the priority list - no one actually used the footage after I handed them the tapes.
But there's a notable exception: Barry Levy's The 40,000, which is a full-length documentary produced with footage I shot for his feature film Spook. It's about the 40,000 Canadians who served in Vietnam, either by being drafted for their dual citizenship (!) or by volunteering. Spook is the dramatized version, and The 40,000 is the doc which illuminates the film.
I was fascinated while shooting, and now I'm transported back to that time by seeing the doc. The real vietnam vet who narrates much of the doc has had his identity obscured to protect him.