Being startled is the result of something near you appealing to only one sense. For instance, suddenly seeing someone without hearing them. Or hearing someone behind you. Or someone grabbing you in the dark. The response is the rest of your senses trying to rationalize what it is - and it is terribly consuming (distracting) untill resolved. The example above (making noise at a distance while approaching) negates the startled response as it helps the brain anticipate seeing someone; two senses detect a person. Distance is also an advantage to prevent being startled. For instance, only seeing someone at a significant distance typically isn’t startling…your brain has time to rationalize the observation without sensing any threat.
If you want to scare someone, appeal to only one of their senses - quietly walk around a corner so they suddenly only see you, say something from a hidden position (around a corner) as they approach so they only hear you, quietly reach out and touch them in the dark when they don’t know you are there…
In contrast, it’s interesting that if at least two senses are aware of a person, we typically are not scared. Making noise at a distance, or getting into a person’s field of vision at a distance are both great ways to help prevent being startled.
Tim