Contractors apply several layers of paint to achieve a suitable finish. They leave the first coat to dry for the recommended time and apply one or more finishing coats. They might choose paints with special qualities for different types of room. Manufacturers have developed paints for bathrooms or kitchens that have good resistance to moisture for example. When they have completed painting, they clear away any equipment and restore the area to its original condition.
We had a bad experience with an interior painter years ago, with the crux of the problem being him overcharging us at the end for "extra work" he didn't anticipate. One thing I'd strongly recommend is making sure it's in the contract that any additional work or growth work is estimated and communicated to the owner as soon as it is identified, otherwise the owner is not liable to pay it at the end.
For interior paint I prefer the semi-gloss, provides a very subtle sheen and is super easy to clean -no flaking, chipping, etc. I've painted a lot, and found that Sherwin Williams Ovation paint has been the easiest to use, and provides the best coverage. I even painted my ceilings with it and WOW! I was done it no time at all, with perfect coverage. House Painting Centennial CO