Mockumentary is a fictional film shot in documentary style — interviews, handheld camera, talking heads — usually for satirical effect.
The mockumentary uses the visual conventions of documentary — handheld camera, observational framing, direct-address interviews, archival inserts — to tell a fictional story, usually comic, occasionally horrifying. Rob Reiner's 'This Is Spinal Tap' (1984) defined the comic strain, building an entire fake heavy-metal band so convincingly that real musicians have admitted recognising themselves in it. Christopher Guest extended the form across 'Waiting for Guffman' (1996), 'Best in Show' (2000), and 'A Mighty Wind' (2003), with largely improvised performances. Television embraced the format with 'The Office' (UK 2001, US 2005), 'Parks and Recreation', 'Modern Family', and 'What We Do in the Shadows'. The horror branch of the genre — 'The Blair Witch Project' (1999), the 'Paranormal Activity' films — uses documentary's claim to truth to multiply dread. The mockumentary works because audiences trust the visual grammar of documentary; subverting that trust is its primary tool, comic or otherwise.