Paper, paint or plaster? Uneven interior walls present a challenge, but there are several solutions to get a fab finish.
The problem with painting over wallpaper is that it will eventually start to come away from the wall, often at the seams. If it has already begun to do this, you can remove the affected sections of wallpaper with a craft knife and cover the missing bits with filler.
You can also simply paint textured wallpaper, but this only works if you like the texture. Textured wallpaper was often used in the past to cover up less-than-perfect walls, so if you do decide to get rid of it, be prepared to do some replastering.
If you can't face the cost and mess of replastering, filler (or, in the worst cases, plaster repair and skim products) can work wonders, but this filling and then sanding can be very time-consuming.
Some walls are beyond repair, unless you line or replaster them, and even then you often have to fill and sand, especially if the plasterer before you has been a bit sloppy.
Newly plastered walls and ceilings can develop hairline cracks, especially over lath and plaster or if the plaster is 'force' dried with heat, in which case lining paper or wallpaper will hold in the cracks. Alternatively, Polycell Flexible Gap Polyfilla (PS2.89, amazon.co.uk) is the best filler I've found.
If you opt for lining paper, paste-the-wall ones are the easiest to use, as you simply apply wallpaper paste to the wall with a brush or roller and then hang the paper. You don't have to grapple with soggy lengths of paper.
While replastering gives a better finish, thick lining paper can be used to improve the appearance of lumpy walls - that said, the lumps and bumps will still be noticeable if light falls across them (lamps are one of the worst offenders).
Blown plaster (plaster that's spongy and moves slightly when pressed on) can also be held in place with lining paper, as long as you don't put any weight, such as tiles, shelves, or coat hooks, on it.
If your walls have seen better days in terms of smoothness, the best way to enhance uneven walls is to use a chalky or flat-matt emulsion. Paint with any kind of sheen will emphasise the flaws.