: Don't jump out the second you see them. Wait until they are at their closest point or just as they turn away from you.

: Corners, behind doors, or just outside a bathroom are classic "low-guard" zones. Wait for them to be preoccupied (e.g., checking their phone).

A guide to Boo's metaprogramming and extensibility features?

If you're looking to give someone a fright, a successful "BOO!" is all about psychology and timing.