Selected seats on BA at T-24, and the flight looked completely dead, so picked an aisle and window for my mother and I. Board, and there is a FA in uniform in the middle seat. We offered to swop, for whichever she would like, explaining that the flight looked empty when we selected the seat. She said 'I have to sit in my assigned seat' (she wasn't on duty!) and she didn't even opt to move to one of the MANY empty seats without a full row (I was right in that the flight was pretty much empty!) It was a bit odd really, but didn't particulaly bother us, we didn't talk over her, my mother read the paper, I watched a movie.