One of the less fun aspects may occur if you hit a point where it was bungled and it's not obvious how to actually make it tick.
When two evil necromancers have taken over the body of a nobleman and a noblewoman -- mortal enemies BTW -- and want to set out on a quest that has nothing to do with what either of the originals would ever have wanted, there's a fair amount of scope in what the firmly loyal guardsmen would do.
Hmm. Perhaps I will have him act normally at first and only later reveal his plans. This will confuse them enough that it will be logical that the nobleman's cousin, holding a higher rank, will be the first to boldly act against him on the grounds that's not really him. The advantages of the habit of command. . . .