If the wall is in poor condition, I will subcontract to a commercial painter to fix the cracks, fill holes, and to apply a primer and deep base coat color.
For new construction, a smooth finish, level 4 is ideal. A primer and base coat should be painted on the wall before the mural work begins.
If it is an existing wall and the wall is in good condition, no special preparation is required.
Here is an example of a before and after image of a close up area on the wall at Crouching Tiger restaurant in Redwood City.