Before applying the first coat of paint, the walls must be clean, dry and smooth. Any areas where filling or priming has been missed will be immediately noticeable once the paint has dried, no matter how expensive the paint is.
To ensure an even finish, work in one go — wall by wall, without long breaks. If the room is large or the ceilings are high, a professional with the right equipment will get the job done quicker and more neatly.