But Plutarch doesn't detail what happened there. He does, however in the Life of Pompey:
52 But when Ariminum was occupied61 and Caesar was reported to be marching against the city with an army, then all eyes were turned upon Cato,
And offers another angle in the Life of Caesar:60 And now word was brought that Caesar had seized Ariminum,79 a large city of Italy, and was marching directly upon Rome with all his forces. But this was false. For he was marching with no more than three hundred horsemen and five thousand men-at‑arms; the rest of his forces were beyond the Alps, and he did not wait for them, since he wished to fall upon his enemies suddenly, when they were in confusion and did not expect him, rather than to give them time and fight them after they were prepared. 2 And so, when he was come to the river Rubicon, which was the boundary of the province allotted to him, he stood in silence and delayed to cross, reasoning with himself, of course, upon the magnitude of his adventure. Then, like one who casts himself from a precipice into a yawning abyss, he closed the eyes of reason and put a veil between them and his peril, and calling out in Greek to the bystanders these words only, "Let the die be cast," he set his army across.
3 As soon as the report of this came flying to Rome and the city was filled with tumult, consternation, and a fear that was beyond compare, the senate at once went in a body and in all haste to Pompey, and the magistrates came too. And when Tullus asked Pompey about an army and a military force, and Pompey, after some delay, said timidly that he had in readiness the soldiers who had come from Caesar, 4 and thought that he could speedily assemble also those who had been previously levied, thirty thousand in number, Tullus cried aloud, "Thou hast deceived us, Pompey!" and advised sending envoys to Caesar; and a certain Favonius, a man otherwise of no bad character, but who often thought that his insolent presumption was an imitation of Cato's boldness of speech, ordered Pompey to stamp upon the ground and call up the forces which he used to promise. But Pompey bore this ill-timed raillery with meekness;80 5 and when Cato reminded him of what he had said to him at the outset about Caesar, he replied that what Cato had said was more prophetic, but what he himself had done was more friendly.
61 Cato now advised that Pompey should be elected general with unlimited powers, adding that the very men who caused great mischief must also put an end to it.
He therefore ordered his centurions and other officers, taking their swords only, and without the rest of their arms, to occupy Ariminum, a large city of Gaul, avoiding commotion and bloodshed as far as possible; and he entrusted this force to Hortensius.4 He himself spent the day in public, attending and watching the exercises of gladiators; but a little before evening he bathed and dressed and went into the banqueting hall. Here he held brief converse with those who had been invited to supper, and just as it was getting dark and went away, after addressing courteously most of his guests and bidding them await his return. To a few of his friends, however, he had previously given directions to follow him, not all by the same route, but some by one way and some by another. 5 He himself mounted one of his hired carts and drove at first along another road, then turned towards Ariminum. When he came to the river which separates Cisalpine Gaul from the prest of Italy (it is called the Rubicon), and began to reflect, now that he drew nearer to the fearful step and was agitated by the magnitude of his ventures, he checked his speed. 6 Then, halting in his course, he communed with himself a long time in silence as his resolution wavered back and forth, and his purpose then suffered change after change. 7 For a long time, too, he discussed his perplexities with his friends who were present, among whom was Asinius Pollio, estimating the great evils for all mankind which would follow their passage of the river, and the wide fame of it which they would leave to posterity. 8 But finally, with a sort of passion, as if abandoning calculation and casting himself upon the future, and uttering the phrase with which men usually prelude their plunge into desperate and daring fortunes, "Let the die be cast,"* he hastened to cross the river; and going at full speed now for the rest of the time, before daybreak he dashed into Ariminum and took possession of it.60 9 It is said, moreover, that on the night before he crossed the river he had an unnatural dream; he thought, namely, that he was having incestuous intercourse with his own mother.61
33 After the seizure of Ariminum, as if the war had opened with broad gates to cover the whole earth and sea alike, and the laws of the state were confounded along with the boundaries of the province, one would not have thought that men and women, as at other times, were hurrying through Italy in consternation, but that the very cities had p525risen up in flight and were rushing one through another; 2 while Rome herself, deluged as it were by the inhabitants of the surrounding towns who were fleeing from their homes, neither readily obeying a magistrate nor listening to the voice of reason, in the surges of a mighty sea narrowly escaped being overturned by her own internal agitations. 3 For conflicting emotions and violent disturbances prevailed everywhere. Those who rejoiced did not keep quiet, but in many places, as was natural in a great city, encountered those who were in fear and distress, and being filled with confidence as to the future came into strife with them; 4 while Pompey himself, who was terror-stricken, was assailed on every side, being taken to task by some for having strengthened Caesar against himself and the supreme power of the state, and denounced by others for having permitted Lentulus to insult Caesar when he was ready to yield and was offering reasonable terms of settlement. 5 Favonius bade him stamp on the ground; for once, in a boastful speech to the senate, he told them to take no trouble or anxious thought about preparations for the war, since when it came he had but to stamp upon the earth to fill Italy with armies.62
6 However, even then Pompey's forces were more numerous than Caesar's; but no one would suffer him to exercise his own judgment; and so, under the influence of many false and terrifying reports, believing that the war was already close at hand and prevailed everywhere, he gave way, was swept along with the universal tide, issued an edict declaring a state of anarchy, and forsook the city, commanding the senate to follow, and forbidding any one to remain who preferred country and freedom to tyranny.
*Another story says that Caesar's actual words were: "Toss the dice high!