On the Sharepoint project I am currently working on I use Nintex Workflow for approval workflows. There is this one list with lots of columns, some of which are irrelevant for certain approvers. So I needed to find a way to hide certain fields from the approver in the approval form (= task form). I googled this a bit, and the solution is actually pretty simple. All you need to do is create a view on the list, called “Workflow Task View”, select the columns you want to be visible and you’re done!

Create a view, hiding the columns you don't want on the approval form
Create a view, hiding the columns you don't want on the approval form
The field is not displayed to the approval form
The field is not displayed to the approval form

Yes, it’s that simple :)