Instructions
Walk up to any of the movie portraits in Barnett College and interact with it to bring up that film's map. This outlines all of the locations and missions available for that film.
Navigate to the mission you want to play and select it. This brings up a menu, where you can choose "Replay Story" or "Free Play." Select the "Free Play" option.
Scroll through the portraits on the "Character Select" menu. This menu displays all of the characters you've unlocked at this point. Pick the main character you want to play as, then pick the support character. Confirm both choices to start the mission.