The character dining check-in is usually to the far left of the entrance turnstiles. Check in with the CM, you can enter much earlier than 5 minutes before your PS. It's great walking down an empty Main Street to the castle.
In Nov. I was a bit disappointed because they let everyone in at 8:45 and roped off all of the lands. We had a difficult time getting into the breakfast because we had to "excuse me" through lots of people waiting for the "rope drop" wondering why we were trying to get to the front. When we did the CM was somewhat rude and made us go to the other side (after working our way through the crowd, stroller and children in tow)--I was not amused. The other CM was much more helpful. This was the only time we ran into this. (we had 9:05 PS and they usually let you in before park opening-but the first CM was being difficult).
The last time we were there for an 8:30 PS in Feb., we had no problem entering an "empty" MK! I'd suggest being there by 8:30, you'll have no problem entering at that time for your 8:45 PS (unless its an EE day)-then it'll be open already...ENJOY!!!