You need to remove all the excess smoke residue from every surface as a starting point. The way to do this is by using a high alkaline cleaner. This reduces the acid in the residues to help prevent ongoing damage.
Don't expect to be able to remove this by using a bucket and solution, as soon as you ring out your first cloth you will have a bucket of oil.
The way to do it is to put the solution into a hand sprayer, spray the wall and wipe off with a cloth, for the ceilings spray the cloth. You need to use a clean section of cloth for each wipe.
The best cloths are terry towell industrial rags available in 10 kilo vacuum packed bundles from a janatorial supplier, 10-15 quid. Don't expect the surfaces to be clean, they will still be black & streaky.
Once you have removed all the excess residue you still can't just go ahead and paint as the remaining soot, there will be some, will bleed through the paint.
The surfaces need to be sealed using a sealer specified for the job. These are available through your normal cleaning suppliers or specialist paint shops.
This is the easiest and most cost effective method, just make sure you are meticulous as any areas missed will damage your resulting re-decoration and could be the source of an ongoing odoour problem.
PS it helps to add a little deodouriser to your alkaline solution.
I have done hundreds of these and never had any problems.
