RE: grayed out field con Workflow when update Case
If these are not the CreatedBy/ModifiedBy fields as Ravi suggested, but custom fields and not Read-Only, then my question is do you have multiple forms for this entity?
Is the form layout that you are seeing in the Workflow Editor that of the form where the fields are read/write?
Is it possible that these are the regular Read-Only fields, and the Read-Write fields are on a different location on the form, or other form.
I would check this scenario, as the Workflow editor displays the default System form, and not necessarily the one you are seeing when you are within the Case Form.
Hope this helps.