The one time this year that I almost had an empty seat next me ...

737-900 - I am in 20A, 20B is empty, and 20C has a guy in it. As boarding is nearly complete, Mr 20C stops a flight attendant and asks "My wife is sitting in the back of the plane, can she move up here to this empty seat?" Flight attendant replies "There would be an upgrade charge." Mr 20C appears to balk at that idea and sort of waves the flight attendant off. The door closes and the flight attendant returns and asks "What seat is your wife in?" Mr 20C replies "36C". Flight attendant goes to the back and moves "wife" to the no longer empty 20B. No payment appears to be made as this all happens in just a few seconds.

Mr 20C and Mrs 20B seem to know each other and talk off and on through the flight.

During the flight, a kid comes up and talks to the lady in 20B. Mr 20C ignores the kid entirely and the kid ignores him in a way that makes me think they don't know each other.

At the end of the flight, Mr 20C bails out without exchanging words with Mrs 20B and heads for the door. Mrs 20B scoots back in her seat to let me out and says to me "I am going to wait for my kids who are sitting in the back, so go past me." I exit the plane and observe Mr 20C far down the concourse heading toward baggage claim at a good pace leaving Mrs 20B and her kids behind.

This all makes it appear like 20B and 20C were not married, and 20C lied to the flight attendant in his successful attempt to get his friend moved up for free.