Find out the reason why they cracked or is it still cracking. YOu wouldn't want to fix it, and have it happen again. Is it a party wall made with brick and perhaps caused by dampness etc. After you determine the cracks are stabilized, (you have fixed the root of the problem), drywall mud will do. Scrape out the crack (make it wider) and mud it. Let it dry, sand, and repeat. After that it should be ready to paint. Or leave the cracks and do a faux type of finish, with an Italian fresco look, depending on the design of the room. Good luck.