If you add some new fields to your actor like double exactX and double exactY you can make your actor move 0.5 cells. Your actor will not realy move 0.5 cells because greenfoot can't set an actor between two fields. But your actor will move 1 cell every two acts. Try using this: When you use double values for the location of the actors and override the setLocation method it should work.

protected is a java modifyer which like public or private. Variables that are declared public are visible from everywhere in your code. A private declaration means that you can not access the variable or method from outside the class they are declared. Protected means that you can't access the variable from outside the class but the variable will be inherited so the methods or fields that are declared protected are visible in all subclasses of the class where the variables are. The variables are also inherited to outside packages. The last modifyer is the default modifyer (when you just declare a mehtod like: void setLocation(...)); The only difference between default and protected is that default is not inherited into outside packages.

Actually, to override a method, it not only needs the same name, but also the same number of arguments and the order of the type of the arguments must also be the same. Therefore, since the method signature 'setLocation(double, double)' is not the same as 'setLocation(int, int)', one does not override the other. Please refer to the java tutorial pages on Classes and Objects, especially to the part on Methods.