As a family of 5 that wanted to stay together on the sleeper from Bordeaux to Nice, we booked the couchette. I had asked the ticket agent when I purchased the tickets, if I could just purchase the 6th bed so it would just be our family in the cabin, and she said no, because the conductor could technically re-sell the ticket if he knew that someone needed a bed. I thought that was strange. As it turned out, an older gentleman ended up in the couchette with us. He seemed very nice, but it was very strange - like sharing a hotel room with a stranger.
Thinking back on this, I know that the conductor never saw 3 out of five us, because we were in the bathroom. My husband just showed them the tickets/passport information.
We will be taking a night train this summer, from Vienna to Venice, and I think I'm going to take a chance and book a "ghost" person for the 6th couchette. If push came to shove, I would say that the person got ill at the last minute and had to go home.
Has anyone else done this before?