In a pre-planned emergency the crew would ask customers sitting by an emergency exit if they were willing and able to help. If you weren't, or the crew decided you would be no use to them, you would be moved from your seat and a more capable person sat in your place.

If it was an unplanned emergency then you have your own conscious to deal with if you took an emergency exit seat and decided to abstain from the responsibility that goes with it!

On long haul aircraft the passengers by the doors would only be expected to open the doors if the crew member in charge of that door was incapacitated!