Given that Claudius now claims to have seen an inscription marked "Poison is Queen" alongside a supposed portrait of Livia, executed a century before her birth, one may question whether, even in the context of the novel, Livia is actually portrayed as a poisoner, or whether this characterisation of her derives from Claudius's imagination. Suetonius describes the historical Claudius as being "cowardly and suspicious," and "terrified by ill-founded reports of conspiracies": unwilling even to attend dinner parties unless "guardsmen armed with lances were stationed around, and soldiers took the place of servants." It is, perhaps, this strand in Claudius's character that Graves is drawing on here.
Marcellus was assumed by many to be Augustus's natural successor. Velleius, in his Compendium of Roman History, tells us that he was "a young man of noble qualities, cheerful in mind and disposition, and equal to the status for which he was being reared." He had served as an Aedile (a junior magistrate) and as a member of the College of Pontiffs (a priestly role), and had begun the construction of the theatre which bears his name.
Claudius points the finger at Livia as bearing responsibility for his death. This suspicion was raised by Cassius Dio in his Roman History, but was rejected by him on the basis of the great number of natural deaths (presumably as the result of some epidemic) that had occurred during the course of that year.
Graves's physical description of Tiberius draws heavily on Suetonius's account in The Twelve Caesars, even down to the strength of his hands ("his left hand was the more agile and powerful and his joints were so strong that he could push a finger through a fresh and sound apple and, with the tap of a finger, could injure the head of a boy or even a youth").
The death described here is that of Nero Claudius Drusus, Claudius's father. He died on campaign in Germany, apparently from an injury sustained when he fell from his horse. Here Claudius once again points the finger at Livia, but with no real evidence to support the contention that Drusus was murdered.