Near the end of the film where the killer holds the pistol in his hand and it follows Bergman's character (Constance) as she leaves the room, they created a giant prop gun near Bergman so that both the extreme close-up of the gun (supposedly in the foreground) and the background action of Constance walking are in focus at the same time.

Hitchcock used a similar trick in "Notorious". About 1:26:00 into the film, there is a scene where Ingrid is sitting in a chair and the poison cup of coffee is sitting in the foreground, camera right. Both are almost perfectly in focus, even though the cup appears to be much closer to the camera (and so it appears about 1.5 times the size of Bergman's head). That is because the cup was actually a prop that was about three feet tall and is placed not far from her face.