The author develops her characters by allowing them each to speak in her own voice in the first person and also by allowing Dede, the preserver of memory in the novel, to speak for them at the beginning and conclusion of the book. In addition, the juxtaposition of the different points of view--Dede, then Minerva, then Maria Teresa, then Patria--allows a more complicated development, a sort of weaving, because we understand the characters through their own eyes / voice as well as through the eyes/ voices of her sisters: it is the combination of all of this that creates the complex development of these women--that is how they become both "deep" and "round," and therefore very real to us. 

Each of the characters goes from a state of innocene to awareness.  Dede is the only surviving Mirabal sister; her sisters, Maria Teresa, Patria, and Minerva are murdered trying to free their country (the Dominican Republic) from the tyrannical rule of Trujillo ("El Jefe").  Maria Teresa goes from being an unquestioning religious woman to a woman full of doubt and less accepting of "pat" answers than she had been.  Maria Teresa goes from a young girl scribbling pictures of dresses and shoes in her diary to designing plans to help the Resistance.  Minerva learns to become less hard and more accepting of human weaknesses.  Dede learns to live with regret and loss and still make something of both her life and the deaths of her sisters in maintaining the shrine to their life and work.