Personalities, contrary to many behavioristic views, are far from exact and precise. Many theorists believe that although personality is somewhat predictable, a portion of of it is inconsistent due to the freedom within all of us, where we respond outside of that which is typical. For example, you may find a person that is relatively sedate, stable and unemotional who is suddenly confronted with a trauma such as being witness to a violent crime. This may bring out aspects of their personality that do not appear consist such as extreme anger and violence.
A big influence of situational behavior is seen in social psychology where personality holds a second seat to social convention, conformity, and even obedience. Stanley Milgram of Yale University showed that behaviors under certain situations can bypass personality entirely, and he consistently demonstrated that even a passive person could be encouraged to do acts of seeming violence at the social direction of a powerful authority's influence.
Solomon Asch also showed that personality could be minimized in situations where one was made to feel foolish, pressured, uneducated or wrong. As a matter of fact, only 25% of people in Solomon Asch's conformity studies were able to express their personalities and self determination when under the negative social pressures of others.
Essentially, personality, though an excellent guide to what may happen, is highly influenced by outside factors, external threats and even internal traumas that emerge from our past. Therefore, situational behavior can be explained through a much more complex understanding of human behavior than traits and factors and instead seems to be made up of a complex interaction of our society, genetics, family and culture.
I hope this helps,