Try reading the link interior prep for the guidelines. If the surfaces are in good shape I have not experienced any special prep for a bath over any other rooms. If hair spray has been used, wash the walls with a non-sudsing detergent as the hair spray acts as a varnish and will cause the paint to not stick well.

Another thing to do, is make sure there isn't any Mold, You can reference this link. <a href="https://www.house-painting-info.com/cleaning-up-mold.html">Cleaning Up Mold on Painted Surfaces</a>

Along with this, make sure you check the old caulking and re apply caulking if it is dry or loose. most commonly above the back splash of the tub or shower where it meets the wall. You may also want to check the sink back splash and the edges of the vanity (Cabinet). Be sure to use a white latex based caulking so the paint will adhere to it.