I have worked on several documentaries, which featured and relied upon one main contributor. I tend to do more in depth assessment and multiple checks, as the individual cannot be replaced, voted off or "hidden" in a group. I use longer interviews (often by Skype, Facetime or phone these days), more clinical personality inventories and evidence from other sources eg health records and family members. Often, contributors are chosen for a documentary because they have certain clinical histories or conditions (eg abuse, self-harming), so the aim is to manage and support rather than exclude.
The Man with No Penis (2014)
Andrew Wardle, born with a congenital condition, faced surgery, after years of "living a lie".
Iraq: Did My Son Die in Vain? (2013)
Geoffrey Dunsmore visited Basra, where his son died in 2007
Branded a Witch (2012)
Kevani Kanda visits the Democratic Republic of the Congo to investigate abuse of children considered to be witches.
The Bigamist Bride (2009)
Emily Horne had married 5 men without divorcing any of them when the programme was made. She seems to have married two more since then.