Sorcery supercedes instant?
In simplest terms possible:  Your opponent can cast any instants in response to your sorcery, but he'll still have to discard 2 cards if he has any.

so, if you cast  mind rot, your opponent can't choose 2 instants to discard and cast them instead.  He can cast instants before he chooses.  

If you cast  duress, your opponent can't cast the instant that you pick for him to discard.  He can cast instants before he shows you his hand.



Here's how It works


You have priority
You cast the sorcery.  
You pass priority to your opponent
Your opponent may cast instant speed spells
You get priority
You pass priority to your opponent
Your Opponent passes priority
The instant spell resolves
You get priority
You pass priority to your opponent
The sorcery spell resolves from start to finish
You get priority
...




Everything that the sorcery spell does happens while it's resolving.  If you look at your opponents hand and choose the cards, than that would happen while it resolves.  If your opponent chooses, he chooses while it resolves.

Once the spell has begun to resolve, You cannot cast any spells until the spell has finished resolving.