For example: what did Urban actually say at Clermont on 27 November 1095 to announce the (First) Crusade? Six accounts have survived.
First, we have a letter Urban himself sent to Flanders. He says "a barbaric fury has deplorably afflicted and laid waste the churches of God in the regions of the Orient" (because he has had a request from the emperor in Constantinople for help with the Turks) and makes a passing reference to Jerusalem by saying the barbarism has "even grasped in intolerable servitude its churches and the Holy City of Christ, glorified by His passion and resurrection." Interestingly, there is no indication that this Crusade has as its main purpose taking over Jerusalem from non-Christians.
There is also the Gesta Francorum ("Deeds of the Franks"), an anonymous history written only a few years after 1095, that simply says Urban called upon people to "take up the way of the Lord" and be prepared to suffer in the undertaking. This account suggests that Urban was calling on the Franks specifically for this task, and caused the Franks to sew crosses onto the right shoulders of their garments to indicate their willingness.
Two eyewitness accounts exist. Fulcher of Chartres was a chaplain whose detailed account of the Council of Clermont (in the week preceding the announcement) gives an account in which he claims to record only things that he saw with his own eyes. He is the best (we think) account of what Urban actually said.
Robert the Monk is the other account. Robert says he was an eyewitness to Urban's speech, and he may have been: Robert has been identified as a former abbot of Saint-Remi who lived from c.1055 - 1122. Writing more than ten years after the speech, he embellishes it (compared to Fulcher's version) and makes it more dramatic. It is Robert who claims that the crowds as one shouted Deus vult ("God wills it!") at the conclusion of the announcement.
Two more accounts that do not claim to have been present exist. Guibert, the abbot of Nogent, adds his own emphasis on returning Jerusalem to Christian possession to fulfill prophecies about the Apocalypse. Baldric, the archbishop of Dol, seems to re-write the account from the Gesta Francorum and emphasize the Crusade as an appeal to chivalry. Part of Urban's focus during the Council was to reign in violence caused by Christian knights in the West.
We take what we can get from the historical record and hope we can assemble the jigsaw puzzle of historical events.
Tomorrow I'll tell you a little more about Guibert of Nogent and his very "modern" skepticism about something that scholarship definitely agrees with, no matter what people in the Middle Ages believed.