Not all that much, honestly, even when you remember that the fiddle wasn't invented until the 16th century! He may have been plucking a lyre when it happened, though: He was giving a dramatic rendition of Troy over in Greece when the fire happened. I actually discussed the events surrounding the fire (and the aftermath) a couple of years ago here, actually! One thing to note is that, despite the quick and immediate disaster relief that Nero began, he followed it up with the monumentally stupid move of deciding to build a monstrous palace right on top of the still smoldering ruins. I'll copypaste my earlier comment here to give that context for you :)
One more quick note of context: Massive fires happened in Rome all the time. This one was particularly large, but they were rather common.
First off, a fire started in Rome. It could have been one of Nero's men (Our sources frequently depict Nero as insane, though that could have just been bias), or it could have been, as he claimed, a Christian (or any religious zealot really) committing an act of terror. Nero himself was out of town when it happened, rumour having it that he was giving a dramatic rendition of the Fall of Troy (Which apparently he was very bad at, our ancient sources do not treat Nero very well). Tacitus also recalls that just before the fire, Nero.... welll.........
He had a raft constructed on Agrippa's lake, put the guests on board and set it in motion by other vessels towing it. These vessels glittered with gold and ivory; the crews were arranged according to age and experience in vice. Birds and beasts had been procured from remote countries, and sea monsters from the ocean. On the margin of the lake were set up brothels crowded with noble ladies, and on the opposite bank were seen naked prostitutes with obscene gestures and movements. As darkness approached, all the adjacent grove and surrounding buildings resounded with song, and shone brilliantly with lights. Nero, who polluted himself by every lawful or lawless indulgence, had not omitted a single abomination which could heighten his depravity, till a few days afterwards he stooped to marry himself to one of that filthy herd, by name Pythagoras, with all the forms of regular wedlock. The bridal veil was put over the emperor; people saw the witnesses of the ceremony, the wedding dower, the couch and the nuptial torches; everything in a word was plainly visible, which, even when a woman weds darkness hides.
That would be what we know today as an "orgy." As I said, these guys didn't like Nero much, but Tacitus does link said debauchery to Nero's insanity, the wrath of the gods, and implies it as "one last roll before we burn the city down" kinda thing.
The fire began in an inflammable section of the market. And considering that Rome was crammed, filthy, dry, there was a strong wind, and the buildings were wooden....it went up like a light. (I'm personally of the opinion that it was an accident, but eh.) The fire spread like...well...wildfire, causing widespread panic and riots (similar to what would happen today if such a massive fire were to break out) - Tacitus writes:
And no one dared to stop the mischief, because of incessant menaces from a number of persons who forbade the extinguishing of the flames, because again others openly hurled brands, and kept shouting that there was one who gave them authority, either seeking to plunder more freely, or obeying orders.
That would be a riot, or mob mentality. I'm still not convinced that Nero had told people to "burn baby burn." As soon as Nero heard about the fire, he rushed back to Rome from Antium and helped the people out - despite the fact that his own palace had burned down too.
Nero at this time was at Antium, and did not return to Rome until the fire approached his house, which he had built to connect the palace with the gardens of Maecenas. It could not, however, be stopped from devouring the palace, the house, and everything around it. However, to relieve the people, driven out homeless as they were, he threw open to them the Campus Martius and the public buildings of Agrippa, and even his own gardens, and raised temporary structures to receive the destitute multitude. Supplies of food were brought up from Ostia and the neighbouring towns, and the price of corn was reduced to three sesterces a peck.
Unfortunately for him....well....the rumour was already running around that the fire was his fault. On TOP of that, it was rumoured that he was comparing Rome to Troy over in Antium. When the fire was finally put out (by making an incredibly massive firebreak - destroying everything in the fire's path, "so that the violence of the fire was met by clear ground and an open sky."), everyone breathed a sigh of relief. And then ANOTHER fire happened and they were all "Oh what the fuck."
Consequently, though there was less loss of life, the temples of the gods, and the porticoes which were devoted to enjoyment, fell in a yet more widespread ruin. And to this conflagration there attached the greater infamy because it broke out on the Aemilian property of Tigellinus, and it seemed that Nero was aiming at the glory of founding a new city and calling it by his name. Rome, indeed, is divided into fourteen districts, four of which remained uninjured, three were levelled to the ground, while in the other seven were left only a few shattered, half-burnt relics of houses.
Here's where Tacitus really implies that Nero was at fault - he gives an excellent breakdown of the damage of the city, and apparently over two thirds of the city was destroyed. That's....insane. That's worse than New Orleans ever had it during Katrina, for perspective (the city still hasn't recovered), and here, Nero embarked on possibly the worst PR campaign ever. He blamed the whole thing on the Christians.
Consequently, to get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Christians by the populace. Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judaea, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their centre and become popular. Accordingly, an arrest was first made of all who pleaded guilty; then, upon their information, an immense multitude was convicted, not so much of the crime of firing the city, as of hatred against mankind. Mockery of every sort was added to their deaths. Covered with the skins of beasts, they were torn by dogs and perished, or were nailed to crosses, or were doomed to the flames and burnt, to serve as a nightly illumination, when daylight had expired. Nero offered his gardens for the spectacle, and was exhibiting a show in the circus, while he mingled with the people in the dress of a charioteer or stood aloft on a car. Hence, even for criminals who deserved extreme and exemplary punishment, there arose a feeling of compassion; for it was not, as it seemed, for the public good, but to glut one man's cruelty, that they were being destroyed.
Yeah, you know it's bad when A Roman who has nothing to do with being Christian, and is used to blood sport, feels sorry for these guys (He doesn't even LIKE them). We don't have any evidence that Christians were "torn apart by wild beasts in the arena," however, being burned alive was the penalty for arson, and crucifixion was considered the cruelest possible death by the Romans.
Then, to compound that blame game, which wasn't going over all that well (Nero was still being blamed for everything), Nero decided that he needed a new palace. And this palace was fucking massive and lavish on a scale that was just silly. Back to Tacitus, because you haven't had enough quotes quite yet!
Nero meanwhile availed himself of his country's desolation, and erected a mansion in which the jewels and gold, long familiar objects, quite vulgarised by our extravagance, were not so marvellous as the fields and lakes, with woods on one side to resemble a wilderness, and, on the other, open spaces and extensive views. The directors and contrivers of the work were Severus and Celer, who had the genius and the audacity to attempt by art even what nature had refused, and to fool away an emperor's resources.
In other words, he built a STUPIDLY lavish mansion over the ruins of Rome, using streets as his measurements instead of normal measurements for houses. Think of what would happen if you used "streets" instead of "metres." The fact that he took advantage of the city being burned down....that didn't go down well at ALL, and really just pushed those rumours farther. For obvious reasons, really. And again, it wasn't the lavishness of the building that really got the Romans. It was the fact that he was literally building this mansion right on top of the ashes of one of the burnt down districts.
To be fair to Nero, Rome was ALSO rebuilt under him - again, a SUPER efficient task, if you really want to think about it. And not only was Rome rebuilt, it was rebuilt BETTER. And what I mean by that is that there were actually building codes - he essentially let a couple of engineers with crazy ideas go nuts. Tacitus DOES praise the post-fire Rome as being better both because it wouldn't become an inferno like the old Rome, and it was more aesthetically pleasing. Nero DID like pretty things.
The reason for the lack of clarity is because rumour, speculation, and hatred are very powerful things.