Whatever the traditional role of the consigliere was at one time, if you look at more recent examples, they're much more hands-on, i.e. running their own crew, directly involved in crimes, etc. I think it's mostly up to the boss but he has to take into account inter-family politics. So, if the boss is from one faction/area of the family, he may give the position to another to keep the peace. Of course, this is if a family is even using the traditional administration model and not simply an acting boss or ruling panel.