Doors are subject to constant use, far more than bare wall surfaces. So we should paint a door to withstand regular use and constant human contact.
A semi-gloss of gloss paint finish on a door is not uncommon. This withstands stains and grease marks better than older matte paints, means that fingerprints are easy to clean off. But modern eggshell finish paint is another possibility because it is also fairly stain resistant.
If the door is being repainted it will need to be cleaned with sugar soap. If the earlier paint is glossy then the surface will need to be slightly roughened. In both cases this is necessary for the new paint to bond to the surface.
It is an advantage to leave doors on their frames, because this allows both sides of the door to be painted in one session. It is a simple matter to slide a protective sheet under the door to protect the floor.
Some doors have raises surfaces, rather like the frame of a painting, or raised panels. These may be painted a different colour to the rest of the door. A brush will be necessary for this type of painting.
If the door has a completely flat surface then it can be painted with a roller. A smaller sized roller is often better for this, perhaps 15cm in width. If you use gloss paint than you will need a roller with a thick nap (the spongy surface of the roller) as this gives a considerably smoother finish.
Small scale projects are fine for DIY enthusiasts. But professional painters will repaint your home or office in minimal time, so you can enjoy the results without effort.