Mural paintings aren't necessary in front of who looks at them, they could be above or all around the spectator, that way it's possible to model space in consequence to prospective or to immagination.
Walls can be decorated or simply hidden behind paintwork as for example an afresco.
Modern colours allow to simplify the making of an afresco.
We can actually paint it on a dry wall instead of a fresh one as in the antiquity.
Trompe l'oeil is an illusion of the eye on a wall or a ceiling or the entire room. A trompe l'oeil could be painted on a piece of furniture or any other object in the order to change it shape and materia into the painted subject.