I made them instance variables to keep them out of method parameters intentionally. It was an OO design choice.

These variables are set up once during the life of the action and not changed. It's part of the design. For example, it's illogical for one method to operate on a different request/context/package/field than another method within the same Action.  I think it would be confusing to make them method parameters - it suggests flexibility that isn't there.