Daniel Radcliffe apparently used a photo of Cameron Diaz as a visual cue for those flying scenes in Harry Potter
Harry Potter star Tom Felton reveals Radcliffe's trick for looking in the right places during shooting

You may have seen a flickering Golden Snitch or a monstrous dragon flying across the screen when watching any of the broomstick scenes in the Harry Potter films, but star Daniel Radcliffe reportedly had his eyes on Cameron Diaz—well, at least a photo of her. In Tom Felton’s new memoir Beyond The Wand: The Magic and Mayhem Of Growing Up A Wizard, the HP star reveals that a young Radcliffe used an image of actor Cameron Diaz as a visual cue while shooting scenes atop a broomstick.
“The approach was that if a piece of magic or any kind of special effect could be achieved practically, that was the best way to do it,” Felton writes (via Insider). “This was especially true in the early days, when the visual effects teams had less-advanced technology at their disposal.”