A silhouette is often created by having a person with zero lighting on him/her, positioned in front of a brightly-lit background. However, you'd need to do camera tests to confirm that the contrast is high enough so you don't see the person's face.
You can emphasize this in Adobe After Effects by creating an oval (face-shaped) mask that is black and partially/mostly opaque, with fuzzy edges. Position this over the mystery person's face and animate it to follow their movements.
There are many other options for hiding a mystery person's face.
- Have the person's back to the camera.
- Only shoot the person from the shoulders down, so their face is not in the shot.
- Only show the shadow of the person's face, on a flat surface that is lightly colored (white, yellow, etc.).
- Similar to the previous idea, the mystery person could be behind a white sheet, with a bright light even farther behind the person, so a silhouette of the person appears on the white sheet. Make sure to reduce or eliminate light on the same side of the sheet as the camera; if there's too much, the shadow will be washed out.
- Position the person behind a sheet of shower glass or something similar so they are visible but indistinguishable.
- Just put a bag over their head.