French overs originally came from the difficulty of getting a camera in front of actors in a car scene. Instead of the kind of typical side coverage you’d shoot of two actors on a couch, French overs capture the scene from behind, so you only get part of the actor’s face in their close-up, and are far from the eyeline.
Even outside of the car scenario, French overs are a great way to shoot a scene — making it seem like the two characters have something to hide. It’s perfect for spy thrillers — or anytime the characters know things the audience doesn’t. We are outside of their personal space, peeking into the conversation and unable to read their full expressions.