Writing a literary commentary is very similar to writing a literary analysis essay in that you will be doing a close reading of the text and analyze it for things such as structure and literary style. But the purpose of a literary commentary is to first identify what effect a specific passage, poem, chapter, or scene has on a reader, and then to analyze how that effect is achieved. To do this you will also need to be very familiar with literary devices, especially devices used to create style, such as point of view, figurative language, rhetorical language, and character development. Below are a few ideas to help get you started.
You want to first think about the main idea or purpose of the scene in question, and there may be more than one main purpose. For example, is the main purpose to simply move the plot forward by creating opportunity for the couple to marry? Or is the main purpose to introduce Friar Laurence and tell us more about his character? Or, perhaps, the main purpose is even to tell us more about Romeo's character. Or perhaps the main purpose is even all three. We certainly learn a lot of interesting things in this scene about Friar Laurence. In particular, we learn that he is a bit unusual for a monk in that he has a vast knowledge of poisons as well as of healing herbs. We also learn a great deal about Romeo's character with our chance to see him through Friar Laurence's eyes. For instance, we learn that the friar thinks him to be very young, foolish, and impetuous, even far too young to know what real love is as we see in Friar Laurence's lines, "Young men's love then lies / Not truly in their hearts, but in their eyes" (II.iii.68-69). To further analyze the purpose of the scene, consider analyzing it for elements of literary style, rhetorical schemes, figurative language, and diction, as well as other literary devices.
Another way in which a literary commentary will differ from a literary analysis essay is in organization. Like an essay, you will have an introduction, and in your introduction you will want to briefly explain the context of the scene, how it fits in with the play as whole. Be sure to only explain things that are relevant and not summarize the play or even the whole scene. However, unlike a typical essay, you will not have a typical thesis statement. Instead, the last sentence of your introduction will briefly state the subjects and themes of the scene. As for the body of the commentary, the easiest thing to do is simply discuss the whole scene "speech by speech, line by line, word by word and sound by sound" ("The Literary Commentary"). Finally, your conclusion will state what your commentary has now shown about the scene that was not otherwise obvious as well as relay the importance of the scene.