Here is a typical happening by Allan Kaprow, the artist credited with having invented the genre:

Maze of wall size mirrors (as at old-time carneys). Rows of blinking yellow blue and white lights. Ouiet neons. Everyone wonders aimlessly. Rubbish on floors in passageways. Five janitors come in with vacum-sweepers sucking up debris. Crackling sounds. Janitors leave. From above, whistling of some sad pop-tune like "Don't Play it no More." More debris is dropped into passageways. Crackling sounds again. Janitors rush around handing out brooms and everybody sweeps. Lots of dust, coughs. Wheelbarrows and shovels rolled in. Frenzied loading of trash, much noise. Brooms are grabbed from people, are held up close to mirrors and examined. Fellow comes in with wide brush and pail of soapy water and wipes over reflections. Janitors sweep and shout at each other from different passageways. But all their words are backwords. They yell louder and faster. Then work and noise wears out and finally stops, dust settles, cans of beer are brought in for everybody. Workman take a swig, burp and pour beer on the floor. They go. Dead silence-------. Three pneumatic triphammers are dragged in. Compressors start. Floor is drilled into, noise is deafening, mirrors shatter.

What may be surmised from this and other happenings is that Kaprow has his finger on the anxiety button, and he seldom lets go. Instead of merging the arts, he merges everyday actions and objects chosen for their ordinariness and anxiety potential. He has been creating happenings since 1958, many on a much more epic scale than Mirrors.