I never startle. OK, well, I did once, just to amuse my younger daughter. She jumped out from behind a door and I pretended to be surprised and she was so delighted that she'd finally *got* me that I didn't have the heart to tell her I actually did know she was there, that I was just pretending.
Sometimes I toss around the idea that other, startleable people are just pretending, too, but the evidence does not support this theory.