The tit-for-tat exchange between Fox News’s Bill O’Reilly and Salon editor Joan Walsh — swapping allegations of having “blood on your hands” after Dr. George Tiller’s murder — made for riveting TV. As Walsh later acknowledged, it was also an example of cable news’ Pee-Wee Herman period — reducing debate to sputtering volleys of “I know you are, but what am I?”

Not long ago, voices on the right warned against speech that might comfort America’s enemies. O’Reilly spoke of bringing treason and sedition charges against Ward Churchill, a former U. of Colorado professor who suggested the Sept. 11 attacks were imperialistic chickens coming home to roost.

With the change in administrations, a strange reversal has occurred. Now conservative commentators are under fire — and indignant about calls to curb their own vehement rhetoric. The slaying of Tiller, a doctor who performed abortions, and a fatal shooting at the Holocaust Museum in Washington inspired liberal commentators to propose that their counterparts are stoking violence. As the website Media Matters argued, “While right-wing media are certainly not legally culpable for any recent attacks, they are responsible for promoting a culture of fear, paranoia and violence that is anti-government in the extreme.”

Since the best defense is a good offense, the knee-jerk if not entirely inaccurate response from the right has been that those on the left were every bit as intemperate and shrill when they were out of power. And oh yes, they’re not responsible for the actions of lunatics.

As is so frequently the case, the two sides continue talking past each other, too dug into ideological bunkers to recognize how their roles have shifted. And while it’s tempting to say a pox on both their houses, the stakes are too high — what with so few appearing to grasp the distinction between urging self-control and actual censorship.

Like a jolt of sanity amid this made-for-cable free-for-all comes “Shouting Fire: Stories From the Edge of Free Speech,” which HBO will premiere on June 29. Directed by Liz Garbus — whose interview subjects include her father, First Amendment attorney Martin Garbus — the documentary could hardly be timelier, examining where the right to shoot one’s mouth off collides not just with the Constitution, but modern reality.

Notably, “Shouting Fire” began during the Bush administration, exploring cases of those punished for speaking out “in the age of terror” and asking, as the filmmaker puts it, “What happens during times of war to free speech and civil liberties?”

Garbus adds that the tables have indeed turned to a degree, with “people on the left saying, ‘Watch what your vitriol brings.’ ”

Perhaps the most interesting comment in the film comes from Churchill, who was fired by the university following the relentless media campaign Fox News and others waged against him. “If (speech) comes at a price … if there are consequences, it’s not free,” he says.

That’s a misreading, however, of what the First Amendment provides. While it protects the right to speak, it’s not a shield against consequences — particularly in the commercial arena, where pressure on third parties, such as sponsors or corporate boards, provide a back-door means to hold speech accountable.

David Letterman is thus “free” to joke about Sarah Palin, which didn’t spare him the headache of an orchestrated protest over his gibes — just as FNC’s trinity of Hannity, Beck and O’Reilly (H.B.O.?) must be prepared to defend the incendiary tone of their increasingly apocalyptic assertions.

The central thrust of “Shouting Fire,” Garbus says, involves “the push toward conformity” — and the delicate question of imposing limits on speech that doesn’t rise to the indefensible standard articulated by Justice Oliver Wendell Holmes of falsely shouting “fire” in a crowded theater.

Very little of the overheated blather tossed about on cable or the blogosphere reaches that threshold, but that doesn’t mean today’s loudmouths can rage on without fear of repercussions — especially if foes can shame or intimidate their patrons into pulling platforms out from under them.

“At the end of the day,” Garbus says of free speech, “you shouldn’t be losing your job over this.”

She’s right, of course — but that won’t prevent critics from trying to make somebody too hot for an employer to handle, even if they haven’t shouted fire.