Inside The Labyrinth (1986)
https://www.youtube.com/watch?v=3LPSOnJyKDA - from the The Fourth Wall YouTube channel (1 hour 56 mins long).

The blurb:
“A behind the scenes look at the 1986 Jim Henson film Labyrinth, featuring David Bowie and Jennifer Connelly.”

One of my favourite making-ofs as it really goes into a lot of details about the filmmaking process (especially the FX part), and unlike most BTS docs, which are usually not more than half and hour or less, this one makes great use of its nearly two hour runtime. Fun little bonus detail: Gates McFadden, who most will recognize as Dr. Crusher from Star Trek: The Next Generation appears in this documentary, though here she’s credited and referred to as Cheryl McFadden, which is the name she used when she worked as a choreographer instead of as an actress.