Show flexfield conditionally if a specific action reason is selected

Summary:

A DFF has an approval workflow when the field is changed. Is there a way to show this field on the condition that a certain action reason is selected?

Content (please ensure you mask any confidential information):

When a HR Specialist is updating a DFF field, can this field (and only this field) be shown when a specific action reason is selected?

E.g.

HR Specialist navigates to Person Management > Employee > Update > Select Action (Promotion) > Select Action Reason (Manager Approver)

HR Specialist updates the DFF named 'Manager Approver' and ticks the box > Review and Submit.

At the moment when the assignment page is open, all fields are visible and avilable for editing. The client wants to restrict this particular action reason to only update the 'Manager Approver' box and no other fields should change. Essentially locking down or hiding the fields in the page.