This was very unusual at the time as the story focused on anti-government issues that showed the rise of a coup d"etat. The story shows a time line of events that were thought to have happened in recent years having to do with the murder of a young Tsarevich by the soon to be Tsar Boris. Although this was after the "real" Boris's time, it was quite unusual for an opera theme to focus on this subject, perhaps it was so unusual, one could call it modern.
The word modern came into much different meaning right around this time of the late 19th century. Many of the leading modernists came from the Catholic church who happened to occupy Moscow during part of Mussorsgy's life. Among the modernists" ideas came something that deeply touched Mussorsgy's music. This was the completely revolutionary idea of realism portrayed in art. As many modernists showed realism in their paintings, Mussorsgy showed realism with his use of time in vocal writing. As the viewer watches the opera unfold, they will notice the plot seems to be moving almost in real time. It is Modest's use of recitative that makes his opera realistic. All of the rhythms, melodies, and vocal lines are very simple. This technique contributes to the seriousness of the words and the story. If Wagner had written "Disaster has come, disaster for all of us, O ye Orthodox," (Boris Prologue) it might not seem so serious or important for the music behind the words would most likely be in the foreground, making the meaning of the words less important. Mussorsgy, unlike Wagner and his predecessors was making music about a drama, rather than a drama about music. .
Mussorsgy realism is further depicted in his musical style. In writing a story about Russian history, the composer decided to use Russian national folk music as the base of his creative style. This influence can also be called evolutionary as opera music before had not used folk music as it was associated with the lower class.