Roxane really existed, she really was Cyrano's cousin, and she really did marry a baron whose surname was De Neuvillette. However, there is no evidence whatsoever that he and Cyrano were both in love with Roxane.
Cyrano's final insult to his own nose is translated as "Oh, that this too, too solid nose would melt." This is a parody of Hamlet's "Oh, that this too, too solid flesh would melt. Gérard Depardieu later appeared in Hamlet (1996), opposite Kenneth Branagh and Derek Jacobi, both of whom have played Cyrano.
The title character was a real historical person. He was born in 1619 and died in 1655. Although there is no real evidence that he actually could compose and recite a poem while dueling, and time his poem so that he could finish the poem and kill his opponent at the same time, he did write the earliest science-fiction books on record, two hundred-fifty years before Jules Verne wrote his. There is also no evidence that he ever wrote another man's love letters.
The historical Cyrano was not murdered. It is true that he got hurt by a log, but he died over a year later, in his cousin's house in the countryside. Edmond Rostand, well aware of these facts, decided for assassination and death in the convent on literary grounds.
Cyrano states that he has "practically lived" Don Quixote. Gérard Depardieu was later considered for the role of Quixote in Terry Gilliam's troubled production, The Man Who Killed Don Quixote. As that role had previously been played by José Ferrer, it would have marked the second role that the two actors shared.
The first act of the original play takes place entirely within the theater, rather than venturing outside, and ends when Cyrano steps out to fight the hundred men sent to kill Ligniere. During his conversation with Le Bret, a food seller offers him some of the concessions. Having just given away his money, by using it to pay back all the theatergoers, he at first refuses, but then accepts nothing more than a glass of water, half a biscuit, and a grape. That detail is not included in this film, since the actions moves outside while Cyrano duels Valvert. However, it does appear to make reference to it. While waiting for Valvert to confront him, Cyrano helps himself to grapes.
With a Best Actor nomination for this film, Gérard Depardieu became the first actor to receive an Oscar nomination for playing the same role in another film version of a work that a different actor (José Ferrer) had previously won the award for. Robert De Niro had won an Oscar for playing Vito Corleone, a role that Marlon Brando had previously won the Oscar for, but De Niro and Brando had played the role in two entirely different films with two entirely different storylines. Ferrer and Depardieu had respectively appeared in different versions of Edmond Rostand's "Cyrano de Bergerac".
Edmond Rostand's original play identifies one character as "a Musketeer." This Musketeer compliments Cyrano after his duel with Valvert, and is later the one who remarks to Cyrano, after seeing him embrace Christian, that they are allowed to talk about his nose again. The earlier scene identifies him as D'Artagnan - which may identify him as the same D'Artagan who is the hero of Alexandre Dumas's novels. Appropriately, both Gérard Depardieu and José Ferrer have appeared in adaptations of the last novel in that series, The Vicomte of Braggelone. Depardieu played Porthos in The Man in the Iron Mask (1998), and Ferrer played Athos in The Fifth Musketeer (1979).
The appearance of the Comte de Guiche in the play and film represents another connection to Alexandre Dumas's Musketeer novels. As referenced in the play, the Comte (Antoine de Gramont III) was married to Cardinal Richelieu's niece. Richelieu was the antagonist in the original Three Musketeers. He also makes a brief appearance in the second Musketeer novel, Twenty Years After. His son, Armand de Gramont, is a major character in that novel and the final one, The Viscount of Bragelonne.