The Personality Psychologists

Personality Psychology is essentially the study of traits. Personality psychologists want to know “What accounts for the differences in individuals and in their behavior?” and laypeople want to know “How can I correctly predict others’ behavior and make judgments accordingly?” Is human nature determined from within or from without? Are our minds and behavior the products of inner forces, or are we shaped and prodded into thought and action by the stimuli of the environment? This is, once more, the classic nature versus nurture question. These debates have always been a part of psychology, since Plato, and likely always will be. Most psychologists now lean toward the interactionist view of the nature/nurture question-that most everything a person does results from an interaction between their personality and the situation they happen to be in. 
Please discuss how we came to this conclusion, specifically when it comes to personality, citing both historical thought and empirical data on both biological and environmental influences. 
And another question… Can we now predict behavior based on personality? How could a layperson go about trying to do this?