Need to make Line manger fields read only during Approval

Summary:

Hi all,

We have a business requirement where when employee raises a Resignation transaction it will go for Line manager approval. The Line manager is then expected to edit and the transaction to provide input in DFF fields. When he edits the transaction then the clients excepts that only the DFF field should be editable and all the other Fields in the When and Why section are non-editable. We have tired to do this by using EL expression but its not working as expected. Please find the screenshots for reference:

Step1 : LM will receive the Notification and he will click on Edit