A friend and I are flying an AC Codeshare from BKK-NRT. Not sure if the fare class or being Star Gold was a factor, but I was able to select my desired seat at time of booking.

At T-24, I was able to move up towards the front of economy and any available seat in the cabin was available to me. My friend who bought the same fare option (different booking reference number) could only pick from a handful of seats - even if they were open to me.