Hecklers are an important part of the dynamics at Speakers' Corner in London's Hyde Park. Some of them have been coming there almost every Sunday for decades. Often they do this just for what they consider fun, but sometimes they have a political agenda of their own.
Generally they try to interrupt and annoy the speakers by shouting obscenities or counter arguments from the middle of the crowd. This particular heckler had a much smarter approach: he was not standing in the crowd but right beside the speaker whom he was heckling. He did not shout at the speaker but rather told the audience how silly the speaker was. His comments became increasingly longer. So eventually he took over completely and the former speaker just stood there silently.