Consider the following scenario:
The Doctor is in a location where there is an Angel present. While walking down a (ahem, well-lit) corridor, he turns a corner and sees an Angel, which is also facing his direction and is now aware of his presence.
The Doctor steps backwards, continuously facing the direction of the Angel. He turns the corner around the corridor and continues to face the direction of the Angel.
Assuming the Angel persues the Doctor, what will happen to it if the Doctor stays still and continues to stare at the corner wall of the corridor?
I ask because this is a bit of a paradox. If the Angel pursues the Doctor, it will freeze as soon as the Doctor sees the Angel. However, angels can't move when they are viewed by another being.
This is a circular problem and I'm curious of this has ever been addressed. Can you see an angel move for the fraction of a second it would take for it to come into your field of vision or is there some other rule that governs this series of circumstances?