The microphone immediately puts a barrier up between the comic and the audience. It establishes the comic as the only legitimate voice in the room, performing the same role aurally that powerful lighting does visually, forcing the audience into silence the same way the lights plunge them into darkness. I think lights and microphone conspire to make the stage experience more artificial, more like watching a film in a cinema than a live person in a room. The audience want to see and hear me clearly, yes, but I also want to see and hear them.
Under some circumstances, I can almost sympathise with the heckler. I don't mean to imply that I have the slightest respect for anyone who maliciously interrupts a performer, and I dislike the laddish, confrontational culture that considers the swapping of insults between stage and bar to be the stock-in-trade of the stand-up's art. But if you're sat at the back in darkness, and the comic is separated off from you, bathed in light, on a platform, voice amplified, having been given a hero's welcome by the emcee, and you're drunk and you're finding it impossible to listen, but you want to be entertained, then wouldn't you naturally become frustrated at the situation?
I think audiences want to feel a connection with whoever's entertaining them. When they heckle, some of the artificial barriers are broken down. The comic has to come out from behind their microphone, shade their eyes from the glare of the lights to make contact with the audience, ditch their prepared lines and start some genuine, spontaneous interaction. It's horrible and aggressive, but it's at least something, something real, something that's actually happening.
I did a stand-up night in Edinburgh that began at 12:30 a.m. The comedians and the audience alike were generally very young and rather drunk, and the microphone was too loud – to help everyone concentrate, I suppose. One comic seemed as frustrated by the evening as the audience were. He slowly peeled off his jokes one by one, but his voice was amped up so disproportionally that, no matter how much the audience laughed, the gap after each punchline seemed silent by comparison. There were only about fifteen people in the room. Increasingly, he began to comment on the apparent indifference of the audience, then started berating the audience for their poor response, and eventually invited them to shout insults at him. A heckle is a better reaction than nothing.
When I got up to perform immediately afterwards, the first thing I did was get rid of the microphone, and ask if everyone could hear me without it. The loud cheer I got in answer to this question was one of the most positive responses I've received to anything I've ever done on stage.