Here is the Annual Pass FAQ from allears.net (a credible source):
Do I have to actually have an Annual Pass before I can call to get any special AP resort rates?
No you don’t. You can call and book the rates without having the Annual Pass in hand. However, if you want to use Disney’s Passholder web site to book at the AP rate online, you will need either an active AP or a voucher to do it. The policy about having it at the time of resort check in is clear as mud. Some people have been allowed to obtain the rate at check in with only their Annual Pass voucher. Others have been told to return to the resort front desk to show the desk CM after activating the pass. Still others have not been asked for any Annual Pass at all. The official Disney policy states an activated adult Annual Pass pass is needed at check in. You may want to avoid any potential embarrassment at check in by having an activated pass at that time.
Source:
http://allears.net/walt-disney-world/wdw-planning/tickets-annual-passes/
I know that nowadays, the CMs can see if you have an activated AP in your account, which is what makes you a Passholder. Everything is completely digital with
MDE. So, there is less of a need to go "show" the AP, they already know whether you have this or not and whether it is activated or not. Whether getting the AP activated midway through the trip is a big deal or not, that we are not clear on. I know that if you arrive late on your arrival day and cannot activate it, it's OK to show it the next day. Not sure what happens if the next day passes. I guess if they ask you for your AP, then you will have to go to Guest Relations to get it activated.