.... Dishonour shall be humour.
O Cassius, you are yoked with a lamb
That carries anger as the flint bears fire,
Who, much enforced, shows a hasty spark
And straight is cold again.

Whereupon Cassius weeps because he thinks Brutus is laughing at him.

Hath Cassius lived
To be but mirth and laughter to his Brutus,
When grief and blood ill-temper’d vexeth him.
Brutus: When I spoke that, I was ill-temper’d too.
Cassius: Do you confess so much? Give me your hand.
Brutus: And my heart too.

Then Cassius explains that he got his temper from his mother (as I did mine).

Cassius: O Brutus!
Brutus: What’s the matter? [Shakespeare should have added `now.‘]
Cassius: Have not you love enough to bear with me,
When that rash humour which my mother gave me
Makes me forgetful?
Brutus: Yes, Cassius, and from henceforth,
When you are over-earnest with your Brutus,
He’ll think your mother chides, and leave you so.

And all this on the brink of disaster and death.

But here comes a rare touch, and we might as well quote it in full.

Mind you, I am following Shakespeare, and not history, which is mostly lies.

A great poet’s instinct might be nearer the truth; after all. Of course scholars know that Macbeth (or Macbethad) reigned for upwards of twenty years in Scotland a wise and a generous king—so much so that he was called “Macbathad the Liberal,” and it was Duncan who found his way to the throne by way of murder; but it didn’t fit in with Shakespeare’s plans, and—anyway that’s only a little matter between the ghosts of Bill and Mac which was doubtless fixed up long ago. More likely they thought it such a one-millionth part of a trifle that they never dreamed of thinking of mentioning it.

(Noise within.)
Poet (within): Let me go in to see the generals; There is some
grudge between ‘em—‘tis not meet
They be alone.
Lucilius (within): You shall not come to them.
Poet (within): Nothing but death shall stay me.