How can I cause red eye in photos I take with my iPhone 6s?

Red eye is the reflection from the retina in the back of the eye. It is caused by the angle reflecting the flash straight back, specifically caused by the flash being too near the lens axis. When the flash is mounted more distant from the lens, the angle is too great to be able to reflect straight back, so the lens does not see it reflected back.

The rough rule of thumb in photography about red eye has always been the flash head should be at least one inch from the lens for every foot of distance to the subject. This is one reason camera hot shoe flashes are built so tall, to increase that distance, to get the flash away from the lens axis.

Regular cameras can place the flash off camera, or can aim it up to bounce from the ceiling. I don't think there's much that be done for cell phones. It will of course help if the subject is not looking back directly at the camera.

How do I cause red-eye in my photos?

• Shoot in the darkest conditions your phone is capable of functioning. Don't select a location so dark that your phone must use an assist light to focus before the shot. The darker ambient light will cause your subjects' pupils to be larger. There are also substances, such as alcohol, nicotine, and others that tend to dilate pupils when consumed.
• Shoot at the longest distance practical for your photo.
• Turn off any "red-eye reduction" options your phone may have. This usually consists of a preflash that has the purpose of constricting your subjects' pupils.
• Place the flash as close to the lens' optical axis as possible.
• Ensure that the phone is not running any sort of "anti-red eye" routine that automatically darkens any red eyes it detects in the resulting photo.