4 ideas
From December 1851 Edmond and Jules de Goncourt kept a single diary in a shared first-person voice, writing up each night the talk and gossip of literary Paris, including the Magny dinners with Flaubert, Sainte-Beuve, Gautier, Renan and Turgenev. When Jules died of syphilis in 1870, Edmond recorded his brother's decline in the same diary and then kept writing it alone until 1896, so the 'we' of the text became an 'I'.
Starting in 1887 Edmond published volumes of the Journal while many of the people in it were still alive. He cut and softened passages to avoid lawsuits, and even so the books provoked quarrels and resentment among those who were depicted.
If you write down a conversation on the night it happens, you catch the offhand phrasing, the malice and the half-formed opinions before hindsight smooths them into a coherent story. The Journal's value as evidence comes from this immediacy. The price is that it also fixes the writers' own spite and prejudices in place, uncorrected by later judgment.
Read the Journal and literary history looks less like a sequence of finished works and more like a running social contest held over dinners, in salons and through rivalries. Reputations get made, tested and wounded in private talk long before any official judgment is formed. The same lens shows the observer too, because the Goncourts recorded others' success with envy that sharpened their eye and also distorted it.