As an illustrator I was trained to turn my paper over to see it backwards and inspect the proportions. Viewing a picture from a different point of view helps you see what your eyes have gotten used to. What looks like a good portrait of a face can be completely warped in reality.
For example take Figure 1
The first drawing is alright, but the second reveals what makes the first one so strange.
Now take this concept and apply it to your scaring. The next time you are in your full uniform get someone to videotape you, being sure to get your dialogue and noises in the audio too. Do your common movement and gestures as if no one was taping you, then go watch the footage. I'm going to bet that more than fifty percent of the time you'll be disappointed by what you see and hear.
I've heard of scare-actors practicing faces in a mirrors, but not often do they think about the over all body movement in terms of, "Do I look like I'm doing what I feel like I'm doing?" The answer is probably no.
After reviewing the footage, remain in uniform and try and create the movement you've been looking for. See what you need to do to change your voice so it comes off the way you want it. I find the best body movement tips and ideas come from good actors playing strong characters.
Here is a list of some actors that come to mind and the movies that I've seen where their character stands out.
Daniel Day-Lewis- There Will be Blood, Gangs of New York
Gary Oldman- Dracula, Leon, Harry Potter and the Prisoner of Azkaban
Ralph Fiennes- Red Dragon, In Bruges, Schindler's List
Jonhy Depp- Take your pick.
Marion Cotillard- La Vie en Rose
Glenn Close- Fatal Attraction, 101 Dalmatians
Kathy Bates- Misery
Charlize Theron- Monster
Angelica Huston- Witches