Two of Viola's characteristics are that she is not only quick thinking, she is also very self-preserving. In other words, she is quickly able to see and understand her vulnerabilities and think of a solution to protect herself. We particularly see this characteristic in the very first scene in which we meet her, Act 1, Scene 2. In this scene, we learn a great deal about her background. Not only has she just survived a shipwreck, we also learn that her father died when she was thirteen and quite possibly her mother before him, leaving her in the guardianship of her brother. But now she feels her brother probably drowned in the shipwreck, leaving her an orphan. More importantly, she is also a wealthy noble woman. Hence, here she is stranded in an unknown land with no family nor male protection, making her very vulnerable due to her wealthy status. It is because she sees her vulnerability that she decides to disguise herself as a servant until she feels it is safe to disclose her true identity. We see her express her desires to protect herself and her wealth in the lines:
O that I served that lady
And might not be delivered to the world,
Till I had made mine own occasion mellow,
What my estate is! (I.ii.43-46)
However, when the sea captain warns her that she would be rejected as a servant in Olivia's household, she next resolves to disguise herself as a eunuch to work for Duke Orsino. Hence, her ability to quickly think of a solution to protect herself not only shows her quick-thinking ability, it also shows she has a self-preserving nature.