Some comments in the Mental Tyranny of Legal Concepts made clear that this is not a broadly known rule, so I thought I'd just mention it here:
In a defamation case, at least when the speech is on a matter of public concern, "the plaintiff [must] bear the burden of showing falsity, as well as [the defendant's] fault, before recovering damages." This is true whether the plaintiff is a public figure or a private figure. See Philadelphia Newspapers, Inc. v. Hepps, 475 U.S. 767, 776 (1986). The common-law rule was that the defendant must prove truth, but Hepps changed that for First Amendment reasons.
Note: Before arguing whether this is the right result, please at least skim the Hepps reasoning.