Just to clarify, you are trying to command them to interact with it, right?
They won't do it themselves, you have to use the "I need you to do something" conversation option.
Also, which NPC are you using?
If they are set as an "essential" NPC, then you cannot sacrifice them. (The essential flag prevents them from being killed.)
If you just want to sacrifice a disposable NPC, there's a guy for hire in the Windhelm inn (Candlehearth Hall) that you can use for this quest.
IIRC, there's a similar one in the Bee & Barb, Riften.
And if you really meant thrall, they won't work as they're already dead once.
Could it be that you are using a mod which sets your followers to essential?
Normally, it should work with any sell-sword that follows you. Also with every non-essential follower, but why wast a good friend if some bad is at hand...
For the most chars does that mean, they run to the next city and take the first sell-sword with them they can find. This means Stenvar from Windhelm.