I haven't been to that event yet. That said, spoken lines can be a hit or miss. BGT teaches improvisational tricks, including picking up on names of guests. You're right. That is one of the most effective ways to creep people out.
The worst thing that can happen with rehearsed lines, is that the actor may sound like a broken record if they don't wait for the next guests. This is particularly a problem at Universal, where they literally wait for a green light that pays no attention to guests. Even KD has trouble with this, especially in Slaughterhouse and Doll Factory.
For me, personally, theatre is everything in a haunt, just like a regular show. If any park knows theatre, it's BGW. They just need to apply their expertise to their haunts a bit more. To some degree, they already did that this year. Both Lumberhack and Scarlett's Revenge (when staffed) had people improvising a scenario at the entrance all night.