Which text did Shakespeare use to base Julius Caesar on? What changes did he make and why?

The primary source for Shakespeare's Julius
Caesar is Plutarch's Lives.  He concentrates on the
Caesar, Mark Antony, and Brutus.

Although Plutarch is his
source, Shakespeare uses the information about these three men to weave a compelling
story about Rome and the change that took place from a Republic to an Empire.  He
doesn't try to pass judgement but he presents the
events.

In a history book, we can read about these events
but Shakespeare puts a human face on the major players.

He
also uses the play to ask important questions.

How does one
stop Caesar from destroying the Republic since the people (mob) want to crown him king? 
Is the majority always right?  Does a war, in this case a civil war, resolve anything? 
Does anybody win?

Shakespeare uses the information gained
in Plutarch to spring board into these and other important
questions.