Is there some one you know who's a little jumpy? This instructable is going to tell you how to take advantage of that and make your victim hopefully give a little scream.
Step 1: Staring
You should probably start with this. While your victim is near or talking to you stare at something stationary, preferably a wall. This will make people feel slightly uncomfortable and prepares them for the next step.
Note: This will take a little bit of time to work and you may have to repeat it to make yourself seem permanently weird.
Step 2: Singing
This is a step that may be combined with staring. Randomly start singing any slightly eerie song, I suggest any oompa-loompa song from the original Willy Wonka movie. It will start to creep people out fairly quickly, it works best with no shown emotion.
Step 3: Weird Conversation
Talk about weird stuff. Some examples are:
Squid- For some reason people don't like them
LiO- It's a slightly disturbing comic
Disturbing jokes- such as "A baby seal walks into a club"
Weapon facts- It just creeps people out
Step 4: Creeping Around
This is the final step and will probably be the most rewarding. It is also very simple: be where the victim doesn't expect you to be. It helps if your in a darker place or on a hard to see object. If done well enough to the right person you could be rewarded with a small scream.
Step 5: Award.
Congratulations. You are now able to creep people out.