The key to approaching any role is to do so without judgment and with a personalized understanding of the character's given circumstances. A character is a combination of facts and circumstances given by the playwright, and the actions the character does to get what they want.
In other words, rather than naming a character with adjectives (i.e., she is negative, she is distant, she is isolated), instead, break down the facts and actions the playwright gives you in the script. This will lead you to a much more nuanced and honest portrayal of a character than trying to come at it from outside judgments.
Remember, nobody believes they are the "bad guy" in the narrative of their life. Every person is acting in what they believe is the best way, in accordance with their situation, needs, and desires.
So, to play Becca, begin by asking:
- What are the circumstances of the play? (Just the facts, not opinions) and how do those circumstances shape my (Becca's) point of view?
- What are the most important relationships between Becca and the other characters? and how does Becca wish her sister would change?
- What does Becca do in the play? These are actions she takes and choices she makes, which are based on her point of view. Examples of actions she might take would be to deflect, to scorch with humor, or to push away (they are always in direct relationship to another character).
Remember that, to every character in a play, there is a story about them in a love relationship with the other character. Find the love between Becca and her sister, and discover a love-based motivation for why she acts the way she does.
The traits that make Becca difficult to play are counterbalanced by the causes that explain them. On the negative side, Becca is isolated and unconnected with other people. She cannot psychologically grasp how people go on with their lives in such a normal way when she is derailed from the normal aspects of life by overwhelming grief and sorrow. In order to find a way to develop the isolation and distance in her character, first fathom the pain and meaning of her grief and sorrow.
Another negative is that her sense of humor is employed against other people rather than to draw others to her and bind a relationship through a shared laugh or private amusing anecdote. Incidentally, this is another one of her traits that further isolates her. To find a way to develop this biting, unpleasant humor in her character, first fathom how small and insignificant daily troubles, pains and sorrows seem to enormous ones: first you must also recognize that troubles and suffering come in degrees of greatness and shades of horror.
Finally, recognize that under her distance, her isolation, and her caustic humor, lies the true essence of her being. On the positive side, she is truly interested in her sister's well-being, even though she nags and criticizes her. She is truly sympathetic of her mother's loss, although she sees that the circumstances of their losses may not compare. She realizes that other people's grief and suffering, especially Howie's, though having different causes (maybe lesser causes) than hers are no less painful and complex than her own: “You’re not in a better place than I am, you’re just in a different place.”