There was a question last month very much like this one, that should answer most, if not all, of your questions. The short version is that in terms of actual events taking place in the book, really nothing is fabricated. Graves addressed criticism from other classicists that he had read over Suetonius and Tacitus, run them together, and made pretty much everything else up by pointing out that pretty much every actual event is attested somewhere in the record. Graves was also perfectly willing to admit that a great deal of his material is from inferior late sources that have pretty much no idea what they're talking about and are obviously fabricating things or misunderstanding them greatly, but he kept it in because it makes for a good story and reinforces the atmosphere of plotting and machination. Graves was also very willing to take Suetonius (who admits that most of what he's writing is gossip and almost certainly has no basis in truth) at face value for plot points. Part of the issue with I, Claudius is that, since Graves was trying to push for a revolutionary new reading of the figure of Claudius as a quite capable and shrewd player in Roman politics, not the bumbling idiot that Suetonius and Seneca paint him as, he's quite willing to believe late sources that attest to Claudius' ups (granted, from the Flavians on Claudius was held as being something of a model emperor) as well as ignore or explain away the evidence that other sources give that paints Claudius negatively. It's the explaining away that's mostly fancy. There's nothing to suggest that Claudius, Livia, Augustus, or any of the other characters thought many of the things that Graves puts in their minds. We know they did certain things, and there are a number of reasons why they might have done so. Graves picks the reasons he particularly likes and crafts a very good story from it, imagining that it is true, whether it is or not. The other thing that Graves fabricates is holes in the record. Graves is very fond of linking events together that probably didn't have any connection--the famous example is the important character of Cassius Chaerea, who appears all over the place and is a major plot-driver. The historical Cassius Chaerea is only known as the prefect of the Praetorian Guard who was hated and teased by Caligula and eventually was one of the leaders of the plot to murder him. Whenever Chaerea appears elsewhere in I, Claudius Graves is in fact imposing his character on a historical person. Basically, whenever Chaerea appears before then he's actually playing someone who the record says was named Cassius, and that Graves assumes or pretends was Chaerea, for plot purposes. There's no reason to suggest, for example, that the same Cassius who led the survivors out of the Teutoburg was the guy who killed Caligula--Cassius was, after all, the name of one of the largest families in Rome. But for more on this, I suggest you take a peep at the other thread.