It's not for the sake of veracity
Whether the historical Caesar pronounced or not the said words is disputed. Of five Antique sources on Caesar's death:
Nicolaus of Damascus, Plutarch and Appian do not report the quote
Suetonius and later Cassius Dio report it only as a dubious variant to the tradition, but using Ancient Greek rather than Latin: Και συ ...
It is left open by the playwright
I am going to look at the three characters you have chosen, starting with Cassius.
Cassius is not an honorable man no matter whether you think Caesar should have died or not. Here is a quote of him trying to use his power to free a friend.
That you have wrong'd me doth appear in this:
You have condemn'd and noted ...
As asked, the question is close to unanswerable. Specifically, your main question is:
What is the evidence - either from the text of the play itself, or from surrounding evidence, if any - of which of these interpretations the writer himself intended?
Gauging authorial intent from the text itself is a fool's game. After all, you've noted that different ...
Some of Shakespeare's plays were printed individually in quarto editions during Shakespeare's lifetime, but Julius Caesar is one of the plays that was first printed after Shakespeare's death in the so-called First Folio of 1623. As a consequence, the First Folio text of Julius Caesar is the only authoritative text of the play, and any variations you see are ...
Shakespeare’s main sources for Julius Caesar were Plutarch’s biographies of Julius Caesar and Marcus Brutus, which he read in the 1579 English translation of Thomas North. The episode you are asking about appears in the biography of Brutus:
His wife Porcia (as we have told you before) was the daughter of Cato, whom Brutus married being his cousin, not a ...
The main sources for Shakespeare's Julius Caesar are three of Plutarch's Lives, namely Marcus Brutus, Julius Caesar and Marcus Antonius.
Shakespeare's use of Plutarch's work is documented in detail in Shakespeare's Plutarch: The Lives of Julius Caesar, Brutus, Marcus Antonius and Coriolanus in the translation of Thomas North, edited by T. J. B. Spencer (...
The discussion of sexual allusions or inferences in Shakespeare has been going on for a century or more. There can be no discussion, though, about the second quotation. (the bawdy hand of the dial is now upon the prick of noon). It's dirty. To answer Knight's question: Shakespeare's trade was theater, not literature. He wrote with his audience in mind, not ...
The phrase et tu Brute was in common use among the Elizabethans before Shakespeare’s “Julius Caesar”. It is a Latin translation of a Greek phrase which Suetonius ascribed to the dying Caesar in his “The Twelve Caesars”.
“Et tu Brute” can be found in a 1584 play in Latin, by Richard Edes, “Caesar Interfectus”; in a 1591 play, “The True Tragedie of Richard ...
The origin of the expression is Shakespeare's Julius Caesar Act III, Scene V:
He was my friend, faithful and just to me:
But Brutus says he was ambitious;
And Brutus is an honourable man.
He hath brought many captives home to Rome
Whose ransoms did the general coffers fill:
Did this in Caesar seem ambitious?
When that the poor have cried, Caesar hath wept:
The most famous study of Shakespeare's "explicit references" is Shakespeare's Bawdy: A Literary & Psychological Essay and A Comprehensive Glossary by Eric Partridge (first published in 1947).
The book contains a 48-page introduction, followed by a 175-page glossary.
I hope it is clear that what is considered taboo or inappropriate varies through time ...
I would paraphrase "rash humour" as irascible temper or fickle temper. Based on what Brutus said earlier, Cassius does not so much have a persisting angry mood; his character is better described as choleric (emphasis mine):
O Cassius, you are yoked with a lamb
That carries anger as the flint bears fire;
Who, much enforced, shows a hasty spark,