Cinema, like any other medium, has a slew of formal and technical rules designed to keep the language of film clear and coherent for viewers.  Every director takes a different approach when deciding which rules to religiously follow, bend, or break altogether.
One such mandate is called the 180 degree rule, which dictates the acceptable placement of the camera for maintaining spatial continuity between two characters in a scene.  To adhere to the rule, the camera must remain on one side of an imaginary line connecting two interacting characters. This is called the axis of action.  Following the rule became standard practice in the Golden Age of Hollywood and it has remained the dominant convention since because it helps orient the viewer.
Just as it’s one of the first rules film school students learn, it is also one of the first rules seasoned directors will break.  Filmmakers will often violate the rule midway through a dialogue scene to emphasize an emotional shift, fractured space, or visually complicate the relationship between characters.  The effect of crossing the axis is often very subtle, but still palpable, if only subconsciously.  When the break occurs, the viewer may notice that the characters have switched to opposite sides of the screen in a sudden and jarring fashion. But even if it goes unnoticed visually, the new spatial relationship in the scene will feel disorienting regardless of whether the viewer knows exactly what’s causing the uneasiness.  For a director, it is a very simple and effective way of impacting the viewer while also tipping them off to the fact that something is amiss.  Check out the video for examples from films.