I will not address procedure in detail because to be frank it’s really subtle and way beyond my knowledge - it is full of technical points in favor of the defendant (intentionally, it makes SLAPPs harder). All I will say about it is that it ends up treated as a criminal rather than civil matter. The plaintiff must porter plainte (press criminal charges), which triggers a criminal procedure, in which they then make a constitution de partie civile (become a civil party to the case), and they get reparation only if the criminal trial convicts the defendant.
The statutory basis is article 29 of the 29 July 1889 law about freedom of the press:
Toute allégation ou imputation d'un fait qui porte atteinte à l'honneur ou à la considération de la personne ou du corps auquel le fait est imputé est une diffamation. La publication directe ou par voie de reproduction de cette allégation ou de cette imputation est punissable, même si elle est faite sous forme dubitative ou si elle vise une personne ou un corps non expressément nommés, mais dont l'identification est rendue possible par les termes des discours, cris, menaces, écrits ou imprimés, placards ou affiches incriminés.
Any factual claim that attacks the honor or standing of another person or group is libel. Publishing or reproducing that claim is punished, even if put as a question, or if the person or group is not readily named but is identifiable [via the context].
Note that "diffamation" (translated above by "libel") is constituted as soon as a claim is factual and objectively disgraceful, regardless of whether the claim is true, or whether it attracts liability. It is common to have the losing instigator of a SLAPP to claim a "victory" against journalists, quoting a part of the verdict where tribunal recognizes the claims to be "diffamatoires" ("libelous"). This sometimes fools the public into thinking that the factual claims were false, even if the tribunal made no such determination, and in fact might have made the opposite determination in another part of the verdict. I will use the English term "defamation" for the common English meaning of "factual claims published in violation of the law" and the term "libelous" for the French legal meaning of "factual claims that attack one’s honor or standing".
Claiming that someone committed a crime is a factual claim that "attacks the honor or standing". That claim need not be written out explicitly, if a reasonable reader would deduce it. That would depend a lot on the exact wording, but "I observe fact A, fact B, fact C, and I have the feeling that all of it leads to the conclusion that Rob murdered Alice" is going to be treated as a claim that Rob murdered Alice.
Now, we move onto the affirmative defenses to defamation. One defense is the exception of truth, where the defendant must prove the allegations to be true or substantially true; however, in the case at hand, they are not going to be able to do it.
Another defense is a creation of case law, the exception de bonne foi (good-faith defense). The general feeling is that it is similar to the US "actual malice" standard in reverse, where the defendant must prove that he or she made significant efforts to verify the truth of the matter, and was measured in their expression. Because it is a creation of case law, it is difficult for a non-expert such as myself to be quite sure of the limits (also, I was told that in recent years, there was a trend towards larger freedom of speech due to ECtHR influence).
An example case would be Cour de Cassation, Chambre civile 2, du 24 février 2005, 02-19.136, according to which good faith requires some objective prudence in the way the assertion is made. A tabloid-style newspaper published an article attacking a singer. A lower-court dismissal was reversed by the Cour de Cassation:
Attendu que les imputations diffamatoires sont réputées de droit faites avec intention de nuire et que cette présomption n'est détruite que lorsque les juges du fond s'appuient sur des faits justificatifs suffisants pour faire admettre la bonne foi ; (...)
Attendu que pour les débouter de leurs demandes, l'arrêt retient que (...) la bonne foi s'apprécie en fonction du genre du journal (...)
Qu'en statuant ainsi, alors que le caractère provocateur et sarcastique du magazine dans lequel avait été publié l'article litigieux ne dispensait pas des devoirs de prudence et d'objectivité, la cour d'appel a violé les textes susvisés ; (...)
Given the presumption that libelous claims are not made in good faith [i.e. good faith is an affirmative defense], and rebutting that presumption requires that justifying elements be given; (...)
Given that to dismiss the case, the verdict holds that (...) good faith is appreciated based on the type of journal [i.e. based on context] (...)
By ruling so, although the provocative and sarcastic nature of the publishing newspaper did not remove the need for prudence and objectivity, the lower court did not follow the law; (...)