The plaster walls in older homes are subject to cracking for a number of reasons. One reason is the natural settling processes of houses and another is humidity and moisture. Minor cracks in the plaster can be touched up with paint, but it is essential to use textured paint for the repair job. Cracks that are more serious require more time and attention. They should be filled with a joint compound and taped before being painted.
Knock off the loose bits of plaster around the crack with a putty knife. Don't be overly aggressive with the putty knife. The goal is to simply knock off the loose bits of plaster. You don't want to make the crack worse.
Sand the area surrounding the crack with medium-grit sandpaper. The edges of the crack may be rough and uneven. Feather the edges with the sandpaper until the area around the crack feels smooth to the touch.
Paint the crack and surrounding area with textured paint. Textured paint is thicker than regular interior paint. It is available in latex or alkyd formulations. Textured paint is often used on bare drywall to hide seams, and is used to hide blemishes and defects in walls and ceilings as well. Textured paint doesn't require a primer. Apply the paint with a paintbrush. Feather the edges around the crack with the paintbrush to help the textured paint blend in with the surrounding areas. Allow the paint to dry. Apply a second coat if necessary.