Alvarin, there's three factors that play into Andrea's mind control, first is how awake they are, second is mental strength, compare Andrea to Angel, Angel is an idoit who Andrea will have no problem in taking over, where Silas could resist and or force Andrea out. And finally physical contact, if Andrea is in direct physical contact with some it'll strengthen the mind controlling process further.
But there are backlashes, first the controlee will be able to hear Andrea's thoughts, since Sam has been controlled twice now, she should be able to hear Andrea's surface thoughts.
