I thought of this.
  1. Open camera/Snapchat
  2. Put thumb on "take photo" button
  3. Put the phone up to ear and say "Hello??"
  4. Turn to the side (90 degrees from target) and pretend to have a phone conversation.
  5. Press your thumb down and take a photo!
  6. You're hidden in plain sight.
  7. Open camera. Turn in the direction of the person you want to take a picture of. Lift phone. Make sure the camera isn't on selfie mode. Make a face like you're taking a selfie. Actually take a picture of the person instead.
    I accidentally did this once with the flash turned on. My cover was blown. It was mortifying.
    Suggested by @celine