I'm sure we all have our ATF's.  I know that I do.  Although I try to treat everyone the same way, with my ATF's I am more relaxed, more myself, I talk to them on the phone longer and more often, if they are so inclined.  I sometimes buy little gifts if I see something that I know they would like.  And even when I don't buy anything but I mention that I was somewhere and I thought of them when I saw something or other, sometimes that seems to touch a nerve, or make us even closer somehow.
I might go to lunch or a meal with them.  Sometimes with the ones that are so inclined I will treat them or cook their favorite meal for them when they come for a session.  I would always give an ATF an extended session, at no charge, but actually, my ATF's are men who, at least at first, started out by really respecting my time and never going much over or they would pay for the extra hour(s).  That is very rare, for me.
Very occasionally, an ATF will spend the night, or we will go away together.  Three months ago, an ATF with all kinds of problems at home and at his business called and asked if he could spend the day just hanging out with me.  I didn't take appts that day and rescheduled one that I had made for that day.  If they are truly an ATF, I would do practically anything for him.

of course we ALL have our favorite clients, the ones with you have orgasms for real because they actually do rock your boat big time.
And, with those, we all have less boundaries.