I’ve took all the tiles off the bathroom walls and the walls are pretty uneven and messy. The tiles I have got are pretty big so I can forsee them being a pita to keep level when the wall is bumpy. Any ideas with what I can quickly skim the walls with to even them out a bit and that will dry quickly? It’s not a shower or bath area either if that makes any odds. Thanks in advance

i thought tiles and tile adhesive were more or less invented for uneven walls and floors – i would just crack on…

scratch coat of unmodified thinset?

Don’t bother. Just get a scraper and take all the high spots off. It doesn’t matter if the surface is not smooth – in fact it’s better if it is not.

Smooth doesn’t matter but it’s a pita if the wall isn’t flat. Take a big flat edge and check the wall. If its roughly ok then remove the high spots as above. If its way out then I would probably use a rough coat of plaster to get it flattish first

If the tiles are porcelain I think the weight means they can pull plaster off and sand and cement skim is preferred. However I didn’t know this and my bathroom still seems ok 6 years on

as above, you have to watch the weight. i’m guesing if they are big they will probably be porcelain(although you can still get large ceramic) and pretty much most porcelain tiles are over the weight that you are meant to tile onto plaster. i think its about 23kg sqm and you also need to allow for the adhesive in that which you’ll use a lot of on big tiles on uneven walls.
in the past i have gone over the weights a little and its been ok but if its a paying customer i’ll refuse to do it and knock the plaster of back to the solid wall.
if you did want to just straigten the existing wall the best thing to use is a rapid setting tile adhesive preferbly non flexible so it doesn’t slump to much. and prime it with a proper primer not pva.