Interviewees generally come in threes.
There’s the Main Guy, the Other Guy, and the PR Bodyguard.
The reason I sit in on client interviews is partly to give moral support to my client and be able to give an after-interview assessment, but also to learn. Each interview goes a little differently and by watching them you are able to better prepare your clients for future interviews.
Note - no client of mine has ever visited a PowerPoint presentation upon a journalist.