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

The field is not displayed to the approval form

Yes, it's that simple :)