Are you asking for yourself or for a child?
Let me share what happened to me. I went to WDW for the first time in almost 20 years in 2000. I found the DIS and got all my questions answered. The result was that I was disappointed in both Test Track <i>and</i> Alien Encounter. Why?
I was disappointed because I knew everything that was going to happen before hand, read glowing reports of both attractions. I <b>expected too much</b>.
What is scary for one person isn't scary for another. One person is thrilled riding in an open car going 64.8 mph on Test Track and another one (like me) comes away from the attraction thinking I had more excitement on I-96 driving 78 mph with the sunroof open.
Let me say this about Alien Encounter. I was not one bit frightened or even startled because I knew <i>exactly</i> what was going to happen ahead of time. AE is the sort of attraction that you really only have one chance at being thrilled, startled or frightened. Once you experienced it the thrill is gone--it's still fun but not quite as much as it was the first time. This attraction depends on the element of surprise for the plot to be carried out.
So, if you are asking because you will have a child with you--it is likely to scare the you know what out of him/her depending on their age. There is a "harness" that comes down hard and tight and there is no wriggle room at all. My nephew was hysterical and couldn't even wiggle out of the seat. A CM had to unlock his seat. If you are claustrophobic be warned and beware.
Most of the show is pitch black which bothers children under 11 or so usually. There are sound effects that can startle adults and frighten children.
I wasn't frightened my nephew was terrified.
Doesn't it drive you crazy the way I just seem to go on and on and on? I hope no one gives you a blow by blow description of AE. Just go and be surprised

Or if you don't like surprises, then I'm sure someone will be happy to share the entire script
