Compare the characters of Rosalind and Celia in As You Like It.
Rosalind starts out with a secondary role demonstrated by the fact that Celia is the one who verbally encourages Orlando while he is wrestling Charles. After Rosalind is exiled by the Duke, she takes charge and makes decisions for her escape to Ardenne Forest and for her safety, plans for which encompass Celia who chooses to go into voluntary exile with Rosalind.
In Ardenne Forest, Rosalind, as Ganymede, a name taken from Greek mythology, is in the male position that is free from anxiety and worry because she is dressed as a man. Rosalind's natural superiority of mind is given full play in her interactions with Silvius, Phoebe and Jaques. One wonders if she would have so bold in speaking her mind and giving instructions if she were still clothed as a wealthy woman.
Though Celia dominates at first, she slips into a quieter, supportive role when she and Rosalind enter Ardenne Forest as Ganymede and Aliena, a role that is necessary to insure their continued safety. Nonetheless, even this quiet role is a powerful one. It is she who buys the cottage and sets up housekeeping. However, since her role and power are quiet and in the background, there is less to say about her.
Comparing these two women brings up the obvious questions of women's safety, women's wit and intelligence, women's ingenuity and capability, and women's friendship. In all of these Celia and Rosalind have equal concern and equal measure. In fact, in regard to safety, they take turns with Celia being protective of Rosalind in the beginning of As You Like It and Rosalind, disguised as the man Ganymede, taking over the protecting role while in Ardenne Forest.
Other less obvious questions are also brought up. One of these questions is the relationship between public power and voice: Can you only have public power if you have the right to speak up, as a man does, as Ganymede does? Another is the value given to silent private power: Is quiet (voiceless) private power of equal value as public power? Another question is whether suppression of the voice (the right to speak up on any or all topics in any or all places) correlates to the suppression of intellect and personality?
(It is interesting to note that some contend that once the women are in Ardenne Forest they are safe and the need for their disguises is dissolved, however, this is an illogical assumption. As we can be see from the play, there are frequent male travelers, there are whole bodies of male exiles, there are poor people who might look kindly at sharing wealth without being invited to do so by way of stealing. There is no reason to think that Shakespeare believed Rosalind and Celia would be safe as women once they were in Ardenne Forest.)
One way to answer this question is to compare the outward affect of the characters, and approach the question as a director or actor might. The two characters are sisters, but it is necessary to have both physical and personality traits that distinguish them.
Rosalind dresses as a boy (Ganymede) but Celia remains in women's clothes, though dressed in rustic mode as a shepherdess, to maintain their disguise as forest dwellers. Rosalind also mentions being "more than common tall" which suggests Celia is shorter; this also suggests a slightly more masculine demeanor in Rosalind, who, as Ganymede, speaks boldly to men and has no trouble convincing others that she is a man.
Celia teases Rosalind/Ganymede when she mentions seeing Orlando, and when Rosalind, regressing to an excited, feminine flurry of emotional excitement, bombards her with questions, she replies: "You must borrow me Gargantua's mouth first. 'Tis a word too great for any mouth of this age's size." This again suggests a physical difference in size between the two, and suggests a casting decision to be made with Rosalind being played by a taller actress.
When Rosalind speaks to Orlando about her sister's love for his brother, she is suggesting parallel connections among the four: "Your brother and my sister no sooner met but they looked, no sooner looked, but they loved," etc. and in this way also emphasizes the immediate attraction between Rosalind and Orlando (and Orlando's more latent "attraction" to Ganymede, portrayed as male bonding).