The beauty of the documentary is in its flexibility.
A documentary could be a series of original interviews mixed with visual reenactments - ala Errol Morris's "The Thin Blue Line" (1988).
A documentary could be a remix of archival footage juxtaposed with voice overs - ala Adam Curtis's "The Century of the Self" (2002).
A documentary could be an immersion into the filmmaker's process making the film itself - ala Nick Broomfield's "Kurt & Courtney" (1998).
Or anything else as yet unimagined.
The plasticity of the documentary makes it infinitely exciting.
But at times it can feel daunting making one: with so many choices, so many movies to live up to, so many different opinions on the Internet, so many excuses not to start...