Personality is a description of the way a person is expected to behave by persons who know them well in response to certain social situations or other stimuli. It is distinguished from behavior, which is a description of how someone acts.
Personality is a core component of mental functioning, and a change of personality can often indicate a serious medical condition affecting the brain, such as Syphilis.
People's personalities are, by definition, predictable. However, they are not static or unchanging. For example, around most people, House is abrasive and abusive, even with his best friend Wilson and his boss Cuddy. However, around his parents, House is reserved and respectful. Similarly, Cameron is usually compliant, honest and non-confrontational. However, in Que Sera Sera, she both stood up to and lied to Cuddy about procedures she had ordered on a patient.
House is very sensitive to changes in behavior that do not match a person's personality. For example, when Cameron started making points in a differential, he immediately pegged her as having read a book on how to negotiate in groups.
However, even House gets fooled. In The Jerk, he thought the patient's personality was a symptom of his disease. Once he eliminated that possibility and accepted that the patient actually was abusive and abrasive, the correct diagnosis came quickly. However, in Insensitive, House recognized the patient's feelings of guilt were actually a symptom of her condition.