What's behind the hard plaster walls?
If your house was built before the 1940's, it's probably a layer of wood lath, followed by a standard 2x4 stud wall with no insulation.  Perhaps some knob-and-tube wiring just to make things interesting.  The plaster will make more dust than you thought possible (wear a mask).

After the 1940's, it's probably gypsum board, but without the foaming agents and plasticizers that make today's drywall easier to use.  Behind it, you probably have a 2x4 stud wall with no insulation and conduit or NM wiring.

[Here's a picture of a lath wall from the back, courtesy Wikipedia]