Not entirely sure I understand the question. I select a value in Field A. What appears in Field B? You say "all the employees in order to select one"? Do you mean it's a checkbox set? Or is it a pull-down list of some sort? Where does it get the list of values - from a related table?
In either case, if all you want to do is clear the contents of Field B when Field A changes, you can do that without a script. Just set an auto-enter calculation in Field B to remove any values that don't belong there. Assuming you have a value list of employees, it might look something like this:
Case ( not IsEmpty ( Field A ) ; FilterValues ( Field B ; ValueListItems ( Get ( FileName ) ; "Employee List" )))
Set the auto-enter calculation to replace the value in the field. Whenever Field A is populated, this will strip out any values in Field B that aren't part of the value list.
Note that this will prohibit any values from being entered in Field B that aren't a member of the value list whenever Field A has a value. If that's not what you're looking for, let us know more details and maybe we can give you a better answer.