Two main themes of "Roman Fever" are developed through the dialogue and characterization of the two main personages, the two "intimate friends" about whom the plot revolves.

RIVALRY

Although Alida Slade and Grace Ansley have been long acquainted and "intimate friends," yet they really know little of each other, probably because Alida Slade has long been jealous of Mrs. Ansley as she knows that Grace has loved her husband, Delphin Slade.

Alida Slade, who always took "conjugal pride" in being married to a business executive, and who is "fuller, and higher in color, with a small determined nose supported by vigorous black eyebrows," observes how Mrs. Ansley, now knitting,

was twenty-five years ago so exquisite lovely, you wouldn't believe it, would you?...though, of course, still charming, distinguished...Well, as a girl she had been exquisite; far more beautiful than her daughter Barbara, though certainly Babs, according to the new standards at any rate, was more effective--had more edge,as they say.  Funny where she got it, with those two nullities as parents....Museum specimens of old New York.

Her own daughter Jenny is such "a perfect daughter that she needed no excessive mothering."  But, Mrs. Slade thinks, with a daughter like Barabara, life would not "be so quiet." She is jealous. And, when she has the opportunity to mock Grace as staid she does, broaching the idea of Grace at a speakeasy--

so amusing that ...she lauched it at a woman's lunch.  It made a hit and went the rounds...

The final evidence of the continued rivalry, of course, occurs as Mrs. Slade divulges that she forged the letter of Delphin to Mrs. Ansley.  On hearing this, Mrs. Ansley drops on one knee:

Mrs. Slade's jealousy suddenly lept up again at the sight.  All these years the woman had been living on that letter.

However, Mrs. Ansley's delivers the coup de grace after her fantasy is destroyed that Delphin wrote to her. Spade may have married Delphin, but Barbara, whom Mrs. Spade is also jealous of, is the her daughter conceived that night at the Coliseum with Delphin.

LOVE and PASSION

Mrs. Slade prides herself upon being more dynamic that the "museum specimen" of Grace Ansley.  She feels that she has contributed much to the dynamics of a prestigious couple that she and Delphin were.  But the sad truth is that she feels more for the son that she has lost, whose death is "agony" to her.  In truth, Mrs. Ansley, who has borne the daughter of Delphin Slade and loved him from across the street quietly, reveals depths of passion beneath the tight corset and high necked dresses that she wears.  As she "dropped to her knee," her face "streaked with tears," after Mrs. Slade's startling revelation, her hidden passion is finally revealed.

Sources:

http://www.enotes.com/roman-fever/themes

http://www.enotes.com/roman-fever/characters

How do plot and character analysis in "Roman Fever" reveal the theme?

One of the main themes is that of competition and "keeping up with the Joneses" mentality of the women in the story.  From the very beginning, Mrs. Slade and Mrs. Ansley are competing for the same man--Mr. Slade.  One ends up marrying him, and considers herself the "winner," but can't understand why she is still envious and jealous of her "friend" who married another man.  It is only in this story set in Rome,where the two are talking and Mrs. Ansley is knitting calmly, that it is revealed that Barbara Ansley, the dramatic and vivacious one (of whom Mrs. Slade is envious since her own daughter is quiet and not so amazing) is the daughter of Mrs. Ansley and Mr. Slade. 

The story gets it's name from a fever caught in Rome if one stays out too late in the night air.  Mrs. Slade, in her youth, wrote a note to entice Mrs. Ansley out into the air in hopes that she would contract the fever and be out of the running for Mr. Slade.  However, the note was supposedly written by Mr. Slade, and unbeknownst to Mrs. Slade, her friend answered the note.  So, the fake meeting actually happened.  The two had a rendezvous, and Barbara Ansley was conceived.  Mrs. Ansley was hurried into a marriage to cover her problem, and the two women have lived in close proximity to one another in the same social circles ever since.

The theme of competition then, is trumped when Mrs. Slade realizes that she didn't really win anything...the vivacious daughter is her husband's by another woman, and Mrs. Ansley seems very happy in contract to Mrs. Slade's misery.

To see the subtle nuances Wharton includes in the story, go back and read it again focusing on the tone of the women's voices, their body language, the silences between them.  They have been forced by circumstances beyond their control since youth to spend time together, yet they don't like one another much at all.