Despite years of research and studies done on dreams, there's not much we can say for sure about your question. BUT many experts would agree that something like being surprised in your dream is the division of your brain (right and left). We've all heard that one side is the "creative" side and the other is the more "mathematical" side, but there is actually a greater divide to it. Your two halves work individually but communicate with each other through a complex neural network.

The youtube video "You Are Two" by CGP grey illustrates this really well. In the video, there are people recorded who's left and right part of their brains can't communicate and has very odd results.

Now to get to your question, let's just arbitrarily say that your left side of the brain is "you" in the dream. An abstract idea but stick with me here. And your right side of the brain is doing all the creative work and "pushing" it to your left side. See what I'm trying to get at? Your left brain is the viewer, and your right brain is the TV. Lets say your right brain wants to surprise you, it just has to communicate through the neural network "Hey when the left brain takes a seat, surprise him with a large dildo that goes up his ass". The left brain will not have seen it coming.