backlight and vocode their voices to disguise their identities, use false names...or halloween masks.
Create a work flow that starts with covering their identities, then overwrites or securely deletes (option empty trash securely on the mac - 37 pass overwrite, better than DOD standards) the original files so their identity can't be discovered even if your hardware/footage is subpoenaed...no tape capture, or destruction of the tapes after import (not just erasing as that can be recovered given enough budget/time).
Make sure your interviewees ttrust you explicitly or you could be in danger depending on the type of criminals/crimes.