Playing a detective who deals with sex crimes is difficult, but facing the possibility of losing her partner, Elliot (Christopher Meloni), was Mariska Hargitay's heaviest challenge yet. ''Lou Diamond Phillips [a murderous kidnapper] has a gun to Elliot's head,'' recalls the actress, 43, about last season's pivotal episode, titled ''Fault.'' ''I'm negotiating [with him to drop the gun]. It was a painful, high-stakes scene. Elliot and I have to admit what we mean to each other.... He is everything that [my character] Olivia has. So this was where we really got that to pay off.''