Film the private moments you feel in your gut that you shouldn't be filming because they're too private.
Robert Greene's "Actress" (2014) is teeming with these moments.
And in Jehane Noujaim + Chris Hegedus's "Startup.com" (2001) there's a wonderful moment where the CEO is praying by himself in his office in midst of his world falling apart.
There's a sense we should not be witnessing this - that this is a moment for their eyes only.
Yet, there it is on screen: and it was so worth capturing.
If your gut is telling you that you shouldn't be filming it, in all honesty, you probably should.