With such an assignment, one needs to consider verisimilitude as the most important element of the letter. That is, the letter must befit the character as protrayed by the author so that it seems genuine. Therefore, one should return to the novel and seek salient personality traits of the character; these are often revealed in dialogue and reactions to the character by other characters. For instance, Dantes is young and ingenuous; having spent much of his time at sea away from his fiancee and her environment, he does not fully comprehend the interrelationships that exist around Mercedes. And, because he is a first mate, his socializing on Morrel's ship is mainly with gentlemen. Obviously, too, his father has protected Dantes from the "seedier side of life." So, he is unacquainted with the type of character that Caderousse is; the deviousness of Danglars is also unknown to him.
With Danglars and Caderousse, also, Dantes's naivete is indicated. In an instance of situational irony, Dantes replies to the greeting of Caderousse, who asks, "are you too proud to speak to them?"
'No, my dear fellow, I am not proud, but I am in love, and I believe love is more apt to make one blind that pride is."
With this blindness of love, Dantes invites Ferdinand, who secretly desires Mercedes, to his betrothal feast. And, when Fernand open his mouth to reply, his voice "died in his throat," but Dantes does not notice his reaction. Yet, he has some premonition for he chastises the men not to speak of him as a captain, for it will "bring bad luck." At the betrothal feast, where his enemies surround him, Dantes does not notice that "Danglars looked at Fernand, who impressionable nature was keenly alive to every emotion," but he does say,
'Joy has that peculiar effect that at times it oppresses us just as much as grief....I cannot help thinking it is not man's lot to attain happiness so easily. Good fortune is like the palaces of the enchanted isles, the gates of which were guarded by dragons. Happiness could only be obtained by overcoming these dragons, and I, I know not how I have deserved the honour of becoming Mercedes's husband.'
Added to Dantes's uncertainly and discomfiture with the forces of fate are his experiences when he is brought before the Deputy Procureur, who initially tells Dantes that he is free to go, but then recalls him, asking to him the captain's letter is to be delivered. At this point, de Villefort sinks into his chair, passing his hand over his brow that drips with perspiration, and rereads the letter a third time (which Dantes does not note). "He made a violent effort to pull himself together." Telling Dantes that the principal charge against the young man is the letter, the Deputy Prosecutor sets the letter afire.
Consider, too, the conversation between de Villefort and Dantes when he is examined:
'Perhaps you have no enemies, but you may have aroused feelings of jealousy....'
'You are right. No doubt you understand men better than I do, and possibly it is so, but if any of my friends cherish any such envious feeling towards me, I would rather not know lest my friendship should turn into hatred.
'You are wrong, you should always strive to see clearly around you....
All that has happened before Dantes's arrest must still be in his mind as he composes his letter to Mercedes, asking her to make every effort to intercede for him, along with his father. Yet, although he may reflect upon these events, one must also consider that Dantes does not recognize the treachery against him until after he meets the Abbe Faria in prison. So, while he may mention some oddities of behavior by the other characters, Dantes will not draw the proper conclusions in his letters because he clearly chooses to be naive, as mentioned in several of the above remarks and quotations.
Good luck, now, with the letter which you will write.