Unless you have recently had your walls plastered you may find that you need to use lining paper on your walls before you paint them. If you are papering them with a thicker paper you may be able to get away without having to line it first.
Lining paper can smooth over the wall, without lining paper you may find that you can see every little imperfection through the paint or paper. Lining paper is relatively cheap but it will add time on to your project as you need to be able to let the paper dry ideally before you then paint it. If the wall is already papered you may not be able to tell if you need to use lining paper until you have stripped it off. Stripping off old wallpaper can cause some of the plaster to fall off the walls, especially in older houses. If it is only a little bit you may be able to use filler, but if a lot falls off or it is starting to crumble then you may need to have it plastered before you can decorate it. It can costs about £200 plus to plaster a room.