I know this was crap but I still have a few questions. (Spoilers)

If Christine and Lex were all in Stacy's head, how did two different people have a conversation with them? First Lex tells the "boyfriend" who is throwing stones at Stacy's window that she doesnt have any friends and later threatens to kill him as he is walking away. If Lex is really stacy, how did that conversation happen? How could the guy not see that is was Stacy? The other secenario was when Christine told the one guy that she recognized him as the one who had raped here. The way he reacts you could see how it was really Stacy that he saw but it's not definitive. Even if a movie is bad I still want it to make some sort of sense. The first scenario definitly didnt.