Final answer:
Cyrano instructs the musicians to stand watch at the corners to ensure Roxane and Christian can converse privately, highlighting his selfless character.
Step-by-step explanation:
The reason Cyrano tells the musicians to stand watch at the corners is to ensure privacy for Roxane and Christian. This strategic move by Cyrano was intended to create a situation where the lovers could speak without being overheard or disturbed by others, particularly by De Guiche, who is also interested in Roxane. By having the musicians stand guard, Cyrano ensures that they can alert him of any unwanted approaches, thus allowing Roxane and Christian to have their intimate conversation in peace.
In the context of the play, this action showcases Cyrano's selfless nature and his willingness to go to great lengths to facilitate the happiness of Roxane, even though he himself is deeply in love with her.