It is often said that truth is a complete defense to libel, but under our system, the person suing must prove falsity to make out a libel claim. Therefore, you need not prove the truth of your statements until the plaintiff has provided a sufficient amount of evidence that it was false. In many cases, proving truth or falsity is not a simple issue.
Fair and accurate reports of government proceedings and documents are usually privileged, but laws vary from state to state. Members of Congress and state legislatures generally have privileges protecting them from libel claims for statements made in the chambers. But, as former Sen. William Proxmire found out when he gave his famous "Golden Fleece" award, when you leave the halls of Congress and make public statements you lose the privilege.
Generally republication of a libel makes the republisher just as liable as the original speaker. However, as communicators, if you republish statements for which the original speaker could not have been sued, you have a privilege against suit.
For a statement to be libelous it must be "of and concerning" the person claiming to have been libeled. Therefore, if you make a comment that "the president of a local bank (not identified) is bankrupt" and there are only two local banks, one of the bankers can sue you for libel, claiming you impugned his business acumen. This concept, know as "group libel," generally only works if the group is smaller than 25 people.
This is, of necessity, a very brief outline of libel. For the publisher or journalist knowing the intricacies of libel law should not be a focus. Rather, they should focus on ensuring the accuracy of what they publish. They should treat every person reported about as though s/he is a private figure who has the greates chance of winning a libel suit, rather than guessing at whether a judge will decide that the person is a public figure. The goal should be to minimize the risk of being sued because, whether you win or lose, a libel suit will cost time and money.
The bottom line is that good prepublication review of your work can save you a lot of time, money and freedom from anxiety.