Sim sessions have an inherent issue that the pilots 'know' something is coming, so they're already 'on the ball'. Running to a 'script' just makes it worse if they already know what's coming.
If there are specific events that they want tested, put in some sort of randomizer that gives them different event scenarios in a random order (the trainer could have a tablet or similar that lets him know what's coming without alerting the pilot being tested).
When Boeing wanted to test some failure scenario, they'd recruit a bunch of random pilots (perhaps pilots that were already there for some sort of sim training) - then hit them with the specific event scenario to see how they reacted. They needed a relatively large number of pilots, since once a pilot had experienced the event once, it was no longer a complete surprise.