Did people support Hitler because they believed in him or out of fear?

Perhaps the most important issue here is the generic term "people." Some Germans strongly supported Hitler, some were opposed to him, and many were indifferent, simply focused on their own personal lives and not involved in politics at all. Each person had his or her own reasons for their particular positions; one can never generalize about all people.
Of the people who did support Hitler, there were as many different motivations as there were individuals. The punitive nature of the Treaty of Versailles at the end of World War I had the effect of fostering German resentment against the Allied powers and contributing to the rise of nationalism and the Nazi party. Hyperinflation and economic frustration also inflamed the anger that led to the rise of the Nazis. 
While some Germans actively agreed with all elements of Hitler's ideology, others simply supported him out of desire for a "strong" leader, nationalism, or economic and personal frustration. Once the Nazis began to gain power, fear of them would have caused some people to pretend to support Hitler to ensure their own personal safety.