The pan needs to be hot when the egg is placed. You need some fat under the egg, the residual oil from the previous cleanup, and the seasoning, helps do this for cast iron. In other nonstick pans you'll almost always have to ensure there is a fat layer between the egg and the pan.

The fat separates the egg from the pan, and allows the egg, once set, to be easily separated from the surface.

Don't go too hot with cast iron, as the heat dissipation from the pan will likely overwhelm an egg. Even my Teflon pans stay right under mid heat when cooking precooked sausage and eggs.

I like to take a few strips of bacon into my cast iron and constantly move them around in the pan, coating the surface and inside with that nice bacon grease. Don't cook the bacon til it's done, just move it around to get the pan greasy, then set aside in case you need that fresh bacon grease again. Cooking eggs in baconf fat or sausage fat adds tons of delicious flavor. And the reason you don't want to cook the bacon too long is because once the fat is rendered down, it dried and becomes brittle, breaking in the pan, dirtying the fatty grease, and if not removed will burn.