Shoot some candid photos.

A candid photograph is a photograph captured without creating a posed appearance. This is achieved in many ways, for example:

when the subject is in motion,
by avoiding prior preparation of the subject,
by surprising the subject,
by not distracting the subject during the process of taking photos.
Thus, the candid character of a photo is unrelated to the subject’s knowledge about or consent to the fact that photos are being taken, and unrelated to the subject’s permission for further usage and distribution. The crucial factor is the actual absence of posing. However, if the subject is absolutely unaware of being photographed and does not even expect it, then such photography is secret photography, which is a special case of candid photography.