It isn’t a really big lie, just a lie to catch your subject off guard. Many people when they see a camera, while they try to act naturally, will glance at the camera out of the corner of their eye. They won’t be as natural as they could be. So, you lie to them. Tell them that when you get to the count of ten, you’re going to take a photo. Then take the photo immediately. They will not be ready because they were expecting you to get to ten. The confused looks on the subjects’ faces are well worth it to continue shooting. You’ll get some great shots, and they’ll be much calmer.
Another small “lie” is to pretend your camera ran out of batteries, or you’re out of memory. Once people think you’re done taking photos, they’ll be au natural, not expecting any more photographs.