Not a liar...misdirecting us for the sake of preserving a plot secret maybe. Remember as we've discussed before this has happened in "Batman Begins" with Ken Wannatabe and Liam Neeson. Can't remember how the two of them handled the press questions about their characters but I think the twist regarding their secret was contained well until the movie's premiered. Unless you were one who read the leaked script you probably wouldn't have known about it until you saw the movie.
It's also very possible that Marion didn't know about her character until she started to film her part. I remember when she was first cast that the question regarding Talia came up on a red carpet event before filming started and she first said no. So this could have changed during production. It's not really a big deal if they're trying to preserve a plot twist.