Why would they put a professional actress in? They're putting in an ex-HM because they actually get on with some of the current HMs as well as not so much with others. It's bound to cause more drama. You could always just imagine that that person was never evicted and then it won't seem so much like a twist.
BB has been getting more and more boring since these 4 got evicted (and indeed one or two others who didn't get a chance) IMO.