I am tiling the walls in my bathroom, not the shower walls, but the rest of the walls. Do I need to install backer board even though it is not in the shower? If so, what thickness and can I install it over the drywall?
The answer is no backer board is not required you can attach tile to sheetrock. However since you are going to the trouble of installing tile you might consider installing backer board. Little things like a pipe breaking in a wall , window / roof leak that wets the sheetrock will spell disaster for sheetrock but a backer board will have no problem in this case.