I'm going to be repainting the interior walls of my house and all of the interior walls have brush marks on them. Will the normal sanding between coats cover up the brush marks or will I have to sand the wall smooth before painting?
If the brush marks are from paint, they will be quite thin. However, if the wall is uniformly covered in similar-looking brush marks, the previous owner may have applied a textured plaster which may be quite thick. Try to determine which is the case and how much effort it will take to remove it by manually sanding in a spot that's not too visible.
Even if it's just paint, you won't be able to remove the brush marks by manual sanding, it would be a huge amount of effort, and the problem with sanding is that it's exercise, which makes you breathe faster, and it also makes a huge amount of dust. This is pretty bad, especially if the paint is lead based.
So if you want to sand it off, please do yourself a favor and rent one of these. I have no idea how it's called in English, it is a vacuum rotary sander typically used to smooth the plaster after installing drywall. This will do the job quickly and without filling your lungs with dust.
Now if you sand too much and reach the cardboard part of the drywall, it will make lots of lint. If you paint over this, you'll get plenty of tiny lumps of cardboard fiber trapped into your paint, and you'll have to sand... again... so it's a good idea to make a test on a part that's not too visible.
Another option is to apply a special thin plaster to smooth the texture. This type of plaster mix is quite liquid and contains surface tension agents which make it a bit "auto-smoothing". You use a squeegee to spread it. I've never used that, but I heard it works.
Are the brush marks from the previous painting very deep? Sanding might be an option if they are very heavy brush strokes.
Another possible solution is to use a heavier nap roller. This will help fill in tiny gaps and is meant to provide better coverage for walls that have a textured surface. But keep in min that the heavier the nap you use on a flat wall the more you will create the orange peel effect. I personally don't mind that but just wanted to point that out.
Be careful that the paint you're sanding isn't old lead-based paint. You don't want to get that in air and breathe that in.
Any way to provide a photo so we can see what you're working with?