Holes and cracks are fairly common on a plaster surface. The main reason is due to movement in the structure or busy areas cracking corners or dented areas caused by moving furniture. Use a scraper to cut into the crack to allow the filler to bond.
Use an interior filler, mix as a paste and apply to crack with the scraper. Push into the crack in all directions from the centre to allow the filler to bond securely. It may be necessary sometimes to cut along a crack close to another imperfection and fill both sections simultaneously.
Once the filler has dried, lightly sand over the surface to even out and blend with the surrounding area.