With Basic Economy and hoards of people being assigned seats the day of now, I don't think there is any hope that you can play games trying to get an empty seat next to you. Best shot is to ask the gate agent if there are any seats with an open seat next to them, but this is highly unlikely nowadays and you have a lot of people asking the same question - gate agents sometimes can preserve an empty next to you, but its unlikely. Best thing to do is just pick a seat you like and are going to be happy with for a 10 hour flight. C&F seats will likely have a little better chance of having an adjacent empty though. For aircraft with 3/3 configuration, people with a travel companion will pick an aisle and window in the same row and hope the middle goes empty so they get the whole row, if not they just switch with the person in the middle so they are seated together.