In greenfoot when I add two objects into the world, the one added first will appear under the one added second. Is there something I can do to an actor to make them always appear on the top layer no matter what?
If they are different classes, you can use the setPaintOrder method of the world: see the docs locally or at http://www.greenfoot.org/files/javadoc/greenfoot/World.html. If they are the same class, there's no mechanism for changing the paint order.