Get and use plaster of paris mix to do plaster repair.
First take a water-soaked rag and hold it in place around the crack or hole that wants repairing. ... until the existing plaster has drawn in as much as it can. Let it soak up the water so that it's pretty damp to the touch but no longer dripping. Then mix up your repair plaster pretty much according to instructions. Not too much water - you don't want it runny but you want it something like pudding. Then trowel in your fresh patch mix. Without the surrounding area being pre-wetted, the old plaster will draw too much moisture out of the setting plaster patch... and when that happens, the new plaster sets too quickly, cracks form in it, and it bonds poorly with the surrounding area. So wet the surround first.
Curing plaster shrinks a bit. You may need to lay in 2 or 3 applications of patch mix to fill the hole. It cures quickly, so prepare only about half a cup or so at a time. Otherwise you'll have most of your patch mix dried and hardened before getting to use it. If you over-fill a crack, then you'll have a chance to scrape down the filled area level with the old plaster if you do so before it sets up real hard. It's awful great fun material to play around with so have some fun out of playing with it.

I've done this type of work alot with drywall mud.
The problem I run into alot is shifting foundations. I'll do this type of repair for a landlord and 6 months later, the cracks are back. I usually give the landlord the choice of repairing it with drywall or with caulking. The drywall looks really nice until it cracks again. The caulking fills the crack and flexes a bit with the shifting, but makes the lines visible if you look close.
Sometimes this is not from shifting foundations, but usually it is. The tell tale sign is when the cracks run in a stair step pattern. If this is the case, also check your' foundation for cracks. They don't always happen together, but you'll always notice cracks in the wall first.