Susan Stamberg wrote about Loopers who, turns out, are background voice artists, not contracted time travelling assassins:

• Loopers provide ambient human voices for public scenes such as those set in restaurants.
• Producers will ask the actors to educate themselves on the location they’re shooting for – they should talk about local sports teams or weather – even if the dialogue is typically inaudible.
• A silent film reel will play on a screen and loopers will mill about a giant microphone speaking as if they were off-screen extras.
• Television shows will usually have around six layers of looper tracks to add real depth to a scene – major motion pictures will have many more.
• Loopers don’t always provide human voices. In Happy Feet and its sequel, loopers were asked to emit the noise that krill – small loud crustaceans – make.
• The most fun looper roles typically involve bloodcurdling screams.

Read more on NPR.