Using IDM 3.5.1 with rbpm 3.6 (I believe:-))

Can someone help me with the following.

I have some mandatory fields on my approval form.
When the approver clicks deny or cancel, the form tells that the
mandatory fields are required.

I would really like it this way.
When "Approve" is selected, the mandatory fields should be required.
When any other button is selected, it should ignore the requirements.

Can someone tell me how to accomplish this? I would not mind a very
specific answer ;-)