Smiling physically alters facial features possibly depending on the intentions of the person smiling.
Most people can fake a believable smile in a photo.
Smiling is harder to fake in a video, because people can see the gestures that occur before, during, and after the smile occurred.
People will likely be more trusting to someone who appears genuine in their body language.
If you want to earn trust and possible involvement from people, try to act naturally in a video.