Digital removal of a reflection in a mirror — composited out or rotoscoped frame by frame. Horror staple, now entirely postproduction.
When the vampire looks in the mirror and nothing looks back — that's the vampire effect in its classic form. You need a character who is physically present in the frame, but whose reflection is missing. This sounds simple, but it's tricky in practice because you're juggling two image planes: the direct character and what the surface should reflect — namely, nothing.
On set, this used to be done with practical tricks: mirrors deliberately angled incorrectly, or gauze mirrors that let light through. Today, you almost always go into post-production. You film the scene normally — the actor sits in front of the mirror, sees themselves. In the edit, you then rotoscope out the reflection and replace it with what lies behind the character: wall, room atmosphere, whatever. Or you recomposite the entire mirror area — depending on how clean the keying work can be.
The critical point: highlights and reflections. If the actor's skin still has a subtle sheen from the mirror, every viewer will notice something is wrong. So, you not only have to remove the reflection, but also check the light continuity of the character themselves — as if the light were actually *not* returning from that mirror surface. Rotoscoping is your friend here, but also your enemy, because it's manual, time-consuming, and prone to errors.
Modern setup: Multi-layer shoot. One camera films the character, a second films the empty mirror (or the reflected environment). In compositing, you then put everything together, mask precisely, and subtly blend out the mirror surface itself to make the missing reflection believable. The effect relies on honesty in the composition — every mistake is immediately visible because our eyes instinctively check mirrors. That's why this isn't a gimmick shot you tick off casually.