You should be aware that cracks in plaster walls are due to building movement, and repairing the crack won't stop the building from moving, so the wall is likely to crack again at the same location and in the same way. Most often you find cracks in plaster walls at the top corners of door ways through interior walls because that's where the plaster wall is weakest. The movement may be due to the soil drying out too much during a drought, or due to frost heave in winter. If the crack is narrow, then the easiest (and probably best) way to fix it is to simply fill the crack with joint compound using a putty knife, and then wiping off the excess joint compound with a damp sponge. That will leave joint compound remaining only in the crack, and you can prime over and paint that crack to match the surrounding wall to yield a near invisible repair. But, it's likely to crack again for the same reason it cracked before.
Anyhow, here's how you would normally repair a plaster wall using modern plastering materials:
You don't really want to go thicker than about 1/4 inch thick with joint compound. If you've got a hole or crevice deeper than that, you want to buy a product called "Base coat plaster" at any place listed under "Drywall wholesalers" in your yellow pages. They'll sell to you if you pay cash.
If you can, try to get Perlite Admix Hardwall by Domtar. I've tried several base coat plasters, and I like it the best.
You paint the surfaces you want the base coat plaster to stick to by diluting white wood glue with enough water to make it into a paintable consistancy. Allow the dilute white wood glue to dry.
You mix your base coat plaster with water (or a dilute solution of white wood glue in water) and apply that to the hole or crevice. Base coat plaster has a lot of body when it comes to bridging gaps because it's meant for being applied to lath. It has perlite in it to give it a lot of bulk without adding much weight. You can pretty well bridge a 2 inch wide (or so) gap with the stuff if you mix it fairly thick. You want to fill the hole or crack to within about 1/8 inch of the surface with the base coat plaster because you have to allow room for the finish coat of drywall joint compound to be flush with the surrounding wall. So, you can apply the base coat plaster until it's within 1/8 inch of being flush with the surrounding wall, OR you can apply base coat plaster so that it's flush with the surrounding wall, wait a few hours for it to stiffen up, and then use a damp sponge to wipe down the base coat plaster until it's about 1/8 inch shy of the surrounding wall. You want the base coat plaster to be stiff enough that when you touch it with your finger, no base coat plaster sticks to your finger. And you use a damp sponge, not a wet sponge, and you squeeze out the sponge in water frequently to clean the base coat plaster off of it. What method you use is determined by the circumstances, and you only learn what works best by experience. You're new to this, so use the latter method because it's probably easier for you to do.
Now, after the base coat plaster dries, apply drywall joint compound over it. (You CAN paint diluted white wood glue onto the surfaces you want the drywall joint compound to stick to OR you can mix some white wood glue into your drywall joint compound. The more white wood glue you add to base coat plaster, the better it sticks and the harder it dries. If you add too much, then you'll see cracks opening up in the base coat plaster as it dries because it shrinks as it dries, and so does white wood glue, so adding white wood glue to base coat plaster increases the amount of shrinkage.
Drywall joint compound also shrinks as it dries. And again, the more white wood glue you add to it, the better it sticks to a surface, but the harder it dries and the more difficult it is to sand smooth. (I've never seen cracks form in drywall joint compound because of adding too much glue, tho.)
And, there are different kinds of drywall joint compound. Obviously there are powders sold by the bag and premixes sold by the pail or cardboard box. And, the powders might have a chemical set to them that kicks in after a certain time. Synko ProSet 90 will have a chemical set that kicks in after about 90 minutes. At that time, the drywall joint compound will, within a matter of a few minutes, become very stiff. It may still be wet, so you can't sand it smooth without gumming up your sanding screen, but you can SCRAPE it down smoother than it is with a paint scraper and apply another coat.
But, beyond that, both powder and premix drywall joint compounds come in three types:
1. "Regular" or "Taping" - Which will have a lot of glue in it so that it sticks well to the drywall and paper tape, but dries hard and that makes it more difficult to sand smooth. You never sand the first coat of drywall joint compound smooth anyhow... you add more coats, and sand the last one smooth. What I do is scrape any lumps or bumps off with a paint scraper on the intermediate coats. I only sand the last coat smooth.
2. "Topping" or "Finish" - which has the least amount of glue in it so that it still sticks as well as it needs to, but dries soft and is easy to sand smooth, and...
3. "All Purpose" which contains a medium amount of glue so that it both sticks OK, but can be sanded smooth without too much effort. This is the compromise between good adhesion and easy sanding, and it's what drywall and plaster repair contractors will typically carry in their trucks so they don't have the additional work associated with carrying around, mixing two different joint compounds and having to clean their tools twice on every repair.
You'd probably want to use All Purpose joint compound over your base coat plaster to get better hardness, but still reasonable sandability.
I've been repairing plaster walls for nearly 25 years now, and I typically use Synko ProSet 90 in the Lite Sand powder (so that it's meant to dry soft as a finish coat) and a gallon of either Weld Bond. By varying the amount of glue I mix in, I can make a compound that'll stick to darn near anything and dry real strong and hard, or a very soft compound that's easy to sand smooth. I like that flexibility, and if there were anything wrong with doing that, I'd'a seen problems with the plaster walls in my apartment block by now. So far, so good. But, but sometimes newbies figure I don't know what I'm talking about because no one else they've talked to has ever suggested adding white wood glue to base coat plaster or drywall joint compound. All I can say is: Try it. Then talk.