How to operate a portrait exposed for 30 seconds?

Exposure for 30 seconds. Nobody moved? It's impossible. This is not scientific. Long exposure generally does not use flash (it may be multiple exposures, first use flash to shoot people quickly, then use slow shutter to shoot the background, and then synthesize or synthesize the body later). The flash should be synchronized with the shutter, generally 250/ s, which is slower than the shutter that should be clicked. There are several kinds of flash, one is to flash people first and then the background to make the front and back clear and bright, and the other is to flash only the foreground-people.