We went on the Inland Passage cruise to Alaska a few years ago with Royal Carribean Cruise Lines. Things have changed since then somewhat, as the cruise ships are largeer and more opulent, but I can answer some of your questions.
A cruise ship is pretty much like a floating hotel when it comes to staterooms and cabins. There are multiple levels and different tiers of luxury. Inside cabins are basic rooms with no porthole or view. Outside cabins have a porthole and view, and the more luxurious cabins may have balconies. The hallways to the cabins are interior and often narrow, just wide enough for two people to pass or to move a maid's cart. Because the cabins are on multiple levels, people tend to use stairs or elevators to access the hallways to their rooms.
The cabins are right next to each other, but the walls aren't particularly thin. However, if there was a loud party or if someone was shouting, you'd probably hear it.
Every cruise ship is required to have at least one safety drill at the beginning of the cruise, and all passengers are required to attend. Pursers will find you if don't show up.
The ship has a gentle roll most of the time, but not enough that feel unsteady on your feet. It's more a sense of slow motion that you adjust to quickly. In the lower levels you can sometimes feel the hum of the engines. We had a storm one night, and the ship was definitely rolling then. In fact, one of our kids felt seasick from the motion.
Most cruises alternate between onboard activities and excursions at ports of call. Large cruise ships have lots and lots of onboard activities, including casinos, stage shows, clubs, shopping malls, swimming pools, water slides, rock climbing, workouts, special interest classes, lectures, etc. My advice would be to look at the websites of some of the major cruise lines, as they often feature shipboard activities. The stage show productions we attended onboard were very professionally done.
Excursions are priced separately from the cruise and can include all sorts of short trips. We went on hikes, went shopping, took a train trip, and took a tour during our excursions. The excursions depend on the location of the ports of call. For example, in Alaska, the excursions feature the natural wonders and the history. A Caribbean cruise would offer different excursions, such as trips to beaches, scuba diving, or parasailing.
Hope that helps a little.