You might want to check Navigators for past sailings on the Dream at
Disney Cruise Line Blog for schedules for your specific itinerary. We did Halloween on the Magic last October so it may be different, but I will say that the 4D showing of Nightmare Before Christmas (followed by the Jack and Sally meet) was on a
separate day from the other Halloween festivities and it was a mid afternoon showing in the Walt Disney Theatre.
I recommend sitting farther to the back. After the movie there is a MAD DASH to the lobby to meet with them and it's a long wait. I want to say we waited about 30 minutes or so to meet them. My daughter was really wanting to go to Friendship Rocks and we were going to miss it, but a very nice group in front of us let us go ahead of them so my DD could get out of there more quickly. (Chivalry isn't dead.)