In short, lord krishna's car was bugged. Sanjay planted a transmitter mic and heard every word spoken in the car. He filtered/added some info to dhritrashtra and then he must have given the script/printout to Veda Vyasa-ji. He put it in meters and metaphors so that it can be remembered and recited easily for good.
Arjun and Lord were actually talking in a casual was like two friends (they were, indeed) were talking.
Dost1 = Meri phat rahi hai ye sab ko dekh ke, mere mama chacha se kaise ladu? Inko kaise maru?
Dost2= Abey saale, tu inko nahi marega to teri aur tere pure khandan ki ye log vaat laga denge. Achchha sun, dont worry, i have latest tech and outdo this guys but before that clarify your mind and believe me.
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Arjuna
Seeing my friends and relatives present before me in such a fighting spirit, I feel the limbs of my body quivering and my mouth drying up.
Chapter 1, Verse 29.
My whole body is trembling, and my hair is standing on end. My bow Gandiva is slipping from my hand, and my skin is burning.
Chapter 1, Verse 30.
I am now unable to stand here any longer. I am forgetting myself, and my mind is reeling. I foresee only evil, O killer of the Kesi demon.
Chapter 1, Verse 31.
I do not see how any good can come from killing my own kinsmen in this battle, nor can I, my dear Krsna, desire any subsequent victory, kingdom, or happiness.
Chapter 1, Verse 32-35.
O Govinda, of what avail to us are kingdoms, happiness or even life itself when all those for whom we may desire them are now arrayed in this battlefield? O Madhusudana, when teachers, fathers, sons, grandfathers, maternal uncles, fathers-in-law, grandsons, brothers-in-law and all relatives are ready to give up their lives and properties and are standing before me, then why should I wish to kill them, though I may survive? O maintainer of all creatures, I am not prepared to fight with them even in exchange for the three worlds, let alone this earth.
Chapter 1, Verse 36.
Sin will overcome us if we slay such aggressors. Therefore it is not proper for us to kill the sons of Dhrtarastra and our friends. What should we gain, O Krsna, husband of the goddess of fortune, and how could we be happy by killing our own kinsmen?
Chapter 1, Verse 37-38.
O Janardana, although these men, overtaken by greed, see no fault in killing one's family or quarreling with friends, why should we, with knowledge of the sin, engage in these acts?
Chapter 1, Verse 39.
With the destruction of dynasty, the eternal family tradition is vanquished, and thus the rest of the family becomes involved in irreligious practice.
Chapter 1, Verse 40.
When irreligion is prominent in the family, O Krsna, the women of the family become corrupt, and from the degradation of womanhood, O descendant of Vrsni, comes unwanted progeny.
Chapter 1, Verse 41.
When there is increase of unwanted population, a hellish situation is created both for the family and for those who destroy the family tradition. In such corrupt families, there is no offering of oblations of food and water to the ancestors.
Chapter 1, Verse 42.
Due to the evil deeds of the destroyers of family tradition, all kinds of community projects and family welfare activities are devastated.
Chapter 1, Verse 43.
O Krsna, maintainer of the people, I have heard by disciplic succession that those who destroy family traditions dwell always in hell.
Chapter 1, Verse 44.
Alas, how strange it is that we are preparing to commit greatly sinful acts, driven by the desire to enjoy royal happiness.
Chapter 1, Verse 45.
I would consider it better for the sons of Dhrtarastra to kill me unarmed and unresisting, rather than to fight with them.
See how big the doubt and fear 'I feel the limbs of my body quivering'=='meri phat rahi hai' can be told by a Maharshi like VV