"Are my videotapes confidental??"
ABSOUTELY! When you mail me videotapes of yourselves, and I evaluate them, I am the ONLY person that sees the tape(s) and they are mailed back to the address of the sender.
"What about confidentality in an interpreter agency??"
The agency send me the videotapes and then when I am done, I put a piece of tape across the open faced end of the videotape. This acts as a "sealing mark" so that when I mail it back to the agency to give it to their employees(such as interpreters, teachers, students, etc)- they can be confident that noone other than the signer has seen the tapes.
Of course everyone is welcome to show eachother their tapes but that is their decision to make.