The more popular actors die last.
A killer is on the loose in a house full of people. He sneaks up behind Bob (who is played by Joe Schmoe) and kills him. He sneaks up behind Phil (another nobody actor) and does away with him too. He then sneaks up behind Alice, but she's played by Angelina Jolie. Our killer tries to kill her but is unsuccessful. In many films this is how it works as the lesser actors (sometimes but not always red shirts) are killed off but not those characters who are played by stars. By the end of the film this tendency has continued and leaves mainly the characters left who are played by well known actors. This does not, however, insure they will not die in the end, but only that they survive longer. This occurs for a few reasons, the main one being that people pay more money to come see a film in which their favorite actor or actress appears in most of it than for one where that character is killed off right away. When subverted it becomes a case of Dead Star Walking. This trope only occurs when the character(s) in question have no other reason to stay alive longer. For example, in the The Bourne Identity Bourne does not survive simply because he's played by Matt Damon, but because he's a trained super agent and is smarter and tougher than the other characters.
As a Death Trope, all Spoilers will be unmarked ahead. Beware.