Rule 8(c) of the Federal Rules of Civil Procedure requires (on pain of waiver) that enumerated defenses be affirmatively pled: accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limtations, waiver, "as well as any other matter constituting an avoidance or affirmative defense." It is the catchall clause at the end that causes difficulties.
The phrasing of the Copyright Act doesn't help matters. Section 106 is the basic grant of rights, and it is very broadly phrased. But there is a catch: Section 106 begins: "Subject to sections 107 through 122...." The Section 106 rights are thus limited by those sections. Those sections begin with something like Section 107 does: "Notwithstanding the provisions of Section 106, the fair use of a copyrighted work ... is not infringement." If something is not an infringement, must it be pled as an affirmative defense?
An affirmative defense is usually regarded as something that comes into play only after plaintiff has made out its prima facie complaint; defendant then says, "OK, but I get off the hook anyway because," the "because" being accord and satisfaction, laches, statute of limitations, etc. The phrasing of Section 107 (and others) seems to say there isn't infringement, which could be construed as referring to the prima facie case or as referring to ultimate liability after affirmative defenses fail.
In Harper & Row v. Nation Enterprises, the Supreme Court held (at my urging) that fair use is an affirmative defense. It has to be pled by defendant, who has the burden of proving it. Other provisions in Sections 107-122 support this reading: the compulsory licenses in Sections 111, 114, 115, 116, 118, 119, which have the same phrasing as Section 107, are classic licenses that FRCP 8(c) regards as affirmative defenses. And treating fair use as an affirmative defense comports with how it is usually tried: the defendant says, "yes I copied, but I get off the hook because my use is transformative, etc."
But what about other "defenses," like joint authorship? One joint author can't infringe the work because joint authors are tenants in common, each owning an undivided interest in the whole. If you are a joint author, that's it, suit over. But is joint authorship an affirmative defense or simply a denial of a critical element of plaintiff's case (unauthorized copying)? I have read two opinions saying joint authorship is an affirmative defense, but I am not so sure.
The difference between a denial of plaintiff's case in chief and an affirmative defense is subtle. With joint authorship, one might say defendant is claiming, "yes I copied, but I get off the hook because I am permitted to copy as a joint author." Or, one could say defendant is claiming, "I can't be deemed to have engaged in unauthorized copying, because by law I can copy." The rubber hits the road when we ask: "Does plaintiff have to prove defendant is not a joint author, or does defendant have to prove it is a joint author?"
That hearkens back to the fair use issue, where the question was whether plaintiff had to prove the use wasn't fair or whether defendant had to prove it was fair. That question has been settled by Harper & Row, and I think correctly. But joint authorship appears, to me, a bit more uncertain.