The question – what causes the dynamic between Cecily and Gwendolen to change – probably refers to the scene in the Second Act when the two women begin to figure out that the situations involving Jack/Ernest and Algernon are not quite what they had initially seemed. Oscar Wilde’s play, The Importance of Being Earnest, is a satire on Victorian social mores, but is also an irreverent comedic depiction of an entirely improbable situation. Jack Worthing is a justice of the peace and the tireless manager of his late stepfather’s estate, and he is guardian to his beautiful 18-year-old “niece,” Cecily. As a temporary escape from the responsibilities of running the vast country estate that was owned by his stepfather, he periodically disappears to London where he pretends to be Jack’s irresponsible brother Ernest. It is as Ernest that Jack has developed a romantic relationship with Gwendolen. In the meantime, Gwendolen’s cousin and Ernest’s good friend, Algernon, has become infatuated with Cecily, who thinks her prospective beau is named Ernest, as that is who Algernon pretends to be in order to woo her. To add to the two mens’ duplicitous natures is Algernon’s invention of an elderly invalid named Bunbury, to whom Algernon must run to provide comfort at a moment’s notice. Algernon, of course, uses the excuse of having to attend to the imaginary Bunbury as a way of extricating himself from socially awkward or dull situations. There is a passage in the First Act in which Algernon summarizes the situation well:
“You have invented a very useful younger brother called Ernest, in order that you may be able to come up to town as often as you like. I have invented an invaluable permanent invalid called Bunbury, in order that I may be able to go down into the country whenever I choose. Bunbury is perfectly invaluable. If it wasn't for Bunbury's extraordinary bad health, for instance, I wouldn't be able to dine with you at Willis's to-night, for I have been really engaged to Aunt Augusta for more than a week.”
The deceptions perpetrated by these two men are bound, given the conventions of theater, to unravel at some point, and it is late in the Second Act when the two women discover that they are both engaged to “Ernest,” not realizing that they are referring to different men, and that neither of those men is actually named Ernest, an important prerequisite for winning the hands of both women, as they both are committed to marrying a man named “Ernest.” The confusion surrounding who’s who leads to friction between the two women, who had, upon initially meeting, taken an instant liking to each other. The confusion that ensues when both women claim to be engaged to Ernest, however, proves too much, as in the following exchange:
CECILY: Dearest Gwendolen, there is no reason why I should make a secret of it to you. Our little county newspaper is sure to chronicle the fact next week. Mr. Ernest Worthing and I are engaged to be married.
GWENDOLEN (quite politely, rising): My darling Cecily, I think there must be some slight error. Mr. Ernest Worthing is engaged to me. The announcement will appear in the MORNING POST on Saturday at the latest.
So, this confusion surrounding the identities of the two male protagonists precipitates a dramatic turn in the new relationship between Gwendolen and Cecily. Their relationship, however, will soon be repaired when they discover that they have both been the victims of the men’s deception, which results in their taking leave of their respective suitors:
GWENDOLEN: I am afraid it is quite clear, Cecily, that neither of us is engaged to be married to anyone.
CECILY: It is not a very pleasant position for a young girl suddenly to find herself in. Is it?
GWENDOLEN: Let us go into the house. They will hardly venture to come after us there.
CECILY: No, men are so cowardly, aren't they?
In conclusion, it is the Second Act when the dynamic between the two women changes from friendly to hostile and then back to friendly, and the developments that spur these changes involve confusion over the true identities of Jack and Algernon.