Usually, when celebrities appear on late-night talk shows, they're there with one sole purpose: to plug their latest movie, television show or album. But leave it to Michael Moore to break the rules. On Monday night, the famed provocateur stopped by "The Colbert Report" not to promote one of his projects, but rather an endangered institution -- the American labor union.

After introducing his guest as "Roger Moore," Colbert dove right in. "Why are public-sector unions attacking Republican governors around the country?"

"I think it's the other way around," (Michael) Moore corrected him.

"That's not how I framed it," the host replied, his lightning wit on full display.

What followed was an entertaining, if not particularly instructive, segment in which Moore cited the many compelling statistics of which he is very fond: The wealthy are sitting on $2 trillion; the wealthiest 400 Americans have more money than the bottom 150 million Americans combined; A person who works at McDonald's pays 3% more in taxes than Stephen Colbert. It almost makes you wish that Moore's interviews came with footnotes.

For the most of the interview, Colbert maintained the ironic guise he does so well, gently ribbing Moore's cultivated "average Joe" image ("Come on, the hat doesn't fool anybody," he joked) but, at about the 5:45 mark, Colbert's interrogation suddenly got more pointed: "Do you believe in the free-market system at all?" he asked.

To see how Moore responded -- and to watch him call Colbert a "tool of Viacom" -- watch the clip above.

As a host, Colbert is intentionally inscrutable, but it seemed as though he was a bit exasperated with Moore. What did you think?

-- Meredith Blake

Twitter.com / MeredithBlake