Generally, plays do not have narrators. How does the fact that Tom is the narrator affect the style and content of "The Glass Menagerie"?
When a play has a narrator, it is, from the get-go, demonstrating its unusual structure. Tennessee Williams was writing during a time when experimental format in plays was not very common; so having a narrator was a bold move. The presence of a narrator signifies the play's meta-dramatic status; this is another way of saying, it is aware of itself as a work of drama. The narrator figure is a de facto playwright, in this way, commanding and controlling the play's events and the pieces of information and action that are revealed to the audience.
In this case, Tom's presence as a character in the play creates an even more complex commentary on the play's structure. His narrator status creates a sort of play within a play (a device first popularized by Shakespeare, who, of course, is the inspiration for Tom's nickname) and one may rightly ask, which is the "real" play? Tom's version of events as he speaks them; or the events that occur as the other characters act them out?
Stylistically, Tom's speech as narrator differs from his speech as character. His "narrator" voice is loftier, more erudite, prone to longer sentences with rhythm and a feeling of poetry in them. The content of his speech allows for him to convey important expository information to the audience, mainly in giving context to the events of the play itself, and their place within the past, the present, or the future. But he is also able to comment upon the play's events and themes in a sort of philosophical way; acting as a sort of dramaturg or even critic. It is a most provocative structure that Williams gave to this play, and is one of the reasons The Glass Menagerie is such an iconic work of America theatre.
Tom is the narrator of the play, and its protagonist; he tells the audience early on that it is a memory play, thus informing the audience that the memories he shares will be from his own perspective. Stylistically, this means that Tom could be considered at least a biased, if not a somewhat unreliable, narrator. His deep unhappiness over the responsibilities that have been foisted upon him cause him to vilify his mother and create pity for himself and his sister, but audiences are savvy enough to understand that there are no heroes, villains, or clear-cut judgments in what emerges as a tragically dysfunctional family dynamic.
It is fair to say that Tom wants us to believe that Amanda and Laura suffer delusions and that he sees himself as an artist who must suffer for his art: he escapes to pursue his writing career but is doomed to carry enormous guilt for abandoning his single mother and disabled sister.
Audiences also are meant to understand that Tom's "writerly" touches inform the play. The perfect symbolism of the glass menagerie, the archetype of the faded Southern belle, and the grandiosity of the drama in such a precise setting are more reflective of Tom's artistic imagination than any realism.
Tom is not only the narrator, but the protagonist of the play. The play is told from his memory. As Tom points out at the beginning of the play, Tom gives us truth "in the form of illusion." Thus, when seen on stage, parts of the play seem rather "dreamy" and, of course, we see all the action from Tom's perspective. We can only guess what the events would look like from Amanda's, Laura's or Jim's viewpoint. If Tom had not been narrating the events, the audience would have never really know the context of the action or the consequences on Tome of leaving his sister.