Something is happening - a performance, an argument, an accident, a spectacle. Don't point your camera at it. Turn around and photograph the crowd watching it.

Faces watching something are more expressive, more varied, and more revealing than the thing itself. This is what Winogrand understood at parades and protests.

Watch for: people who aren't watching - the one person in the crowd looking the wrong way, distracted by their phone, bored, disconnected. That's often the picture.