As of tomorrow Grand Theft Auto 5's PC version will have its own built-in movie editor, allowing players to remix the game's various assets into short films.
Dubbed the Rockstar Editor, this video editing tool allows players to capture footage they're about to pull off, or just pulled off, then edit it in various ways. They can alter the camera modes, mess with filters and depth of field, select whatever in-game music they prefer, and more.
There will even be a "Director Mode" that lets players tinker around with various in-game assets just for making films. This will allow you to play as various main characters from the story mode, pedestrians and even animals, so you can make them "act" in your film. This sandbox mode even lets you change the weather and time of day to best capture the mood you want.
Whatever in-game movies you concoct can then be uploaded directly to YouTube and the Rockstar Games Social Club.