This is just a placeholder post for now, something I'll add to at a later date. There's no substitute for viewing (though reading the likes of Stephen Rebello's Alfred Hitchcock and the Making of Psycho , or any of the many journal/web articles devoted to this topic will certainly help) examples to boost your understanding of how filmmakers maintain verisimilitude in this central feature for slasher audiences.
Sometimes, though, there's an intentional degree of campness, even before Scream made postmodernism part of the slasher mix. The vid below (a daft vodcast ... with over 175k views) contains some examples, but its not hard to find many more on YouTube.
NB: The following vid features strong language and clips from 18-rated films featuring strong violence.