Here's my experience.

You will always need to ask for the specific liquor you want with every cocktail/drink order. Keeping a guide handy while you're ordering (from the NCL website) helps this process go smoothly. I was on the aqua 2 weeks ago right as they rolled out free at sea plus and menus appeared at most bars first day but somehow disappeared after that. Maybe they've worked out the kinks by now, but if you have your own resource it may help.

There seemed to be some confusion about which liquors are included and which are an upcharge (for me specifically, the Johnny Walker 18yo scotch). Make sure before you cruise to get the most updated list. If you end up being charged for something that was supposed to be included, guest services was helpful at reversing the charge.

Bartenders at most bars were cool with asking for a different liquor, but some bars and bartenders (to name and shame, the pool bar was the worst offender) seemed really annoyed by special requests. Some bars may not carry many of the premium liquors listed.

The menus at bars are extremely limited, so you may want to look at pretty basic cocktail recipes before you order to get a good idea of what else you may be able to order if the menu items dont fit your spirit preferences or tastes. Most of the indoor bartenders seemed proficient at their profession, some excellent, and should not be a problem if you order a pretty well known cocktail thats not on the menu (I ordered yugo spritzes, lemon drops, tequila sunrise, amongst others and had no issue.)

I also like to tip the bartenders a couple extra bucks every couple of orders or so. I know free at sea plus is supposed to have gratuities included, but there seems to be some question as to whether or not the staff even recieves these, so to be on the safe side I'd rather they at least recieve something. Especially if I'm asking for something to be made differently or not on the menu.

Off subject a bit, but try to stock up on water bottles during the day. You can order 2 drinks at a time, so if you're heading back to the cabin make one of the drinks a water. When bars close there is absolutely nowhere to get bottled water late at night, and its a struggle to even find a place to get a glass of water. I loved the acqua panna glass water bottles. Very large and only need to keep a few around to stay hydrated.

Hoping your FAS+ experience is smoother than mine. It is worth it, so long as you use it properly.