Prime Minister, who heads the executive branch, is the most powerful and important figure in government?

The contemporary parliamentary system is characterized by prime ministerial power. There is no doubt that the Prime Minister, who heads the executive branch, is the most powerful and important figure in government. Questions for you to answer:
 What are the sources of prime ministerial power?
 Are there effective constraints on these powers of the prime minister?
If so, what are they: the Governor General? Parliament? The media?
The judiciary? The PM’s own party? Any other?
 Or, is the PM in effect a “friendly dictator,” as journalist Jeffrey
Simpson once claimed?