To photograph events (holiday crowd, street festival, and wedding etc.), there are two approches, the traditional portraiture and photojournalism. Photojournalism documents an event without interfering or influencing the images. My understanding is that you must push the shutter button at just the right moment to caputure the emotion and essence of your subject. The subject may already be aware of you are shooting at that moment, however, his/her face and body don't have enough time to react, or say pose for you yet. That one to two seconds make big difference.
Look at this photo I took about 4 seconds after #4. Right after I took #4, the guy noticed me, and the same time, another guy jumped in (Canon 300D with the 17-40mm4L big eye, ), so he did a frozen un-natural posture on purpose. It was a funny posture, but, the moment I wanted was gone.