How to sacrifice someone in Boethiah's Calling
I am at the point where I am to "Lead someone to the shrine and slay them."
I have a companion following me and have lead them to the shrine but nothing is happening. What am I supposed to do? Does it only work with certain NPCs?
It could be one of two things: The follower you're using, or what you're doing when you get there.
I noticed some of the followers wouldn't do what you needed them to do. One example being one of the Dark Brotherhood initiates. I've found it works best with one of the mercenaries you can hire from one of the main cities. I used the one in Solitude. A nice list put together by the guys at SegmentNext can be found here. It shows where they are and what their names are.
When you get to the shrine with your follower, you want to talk to them and use the option "I need you to do something." Your cursor will then change, as will the action you're performing. Simply look at the shrine, and it should say "Activate sacrifical altar" or something along those lines. An animation should show your follower walking to it, and then being attached to it. From there, simply kill them by any means, the Sacrificial Dagger works, but is not required.