Jim Garrison had a lot of power. And he was determined to use his powers to achieve hjs goals. One of his biggest was his power of subpoena. One of his favorite techniques was to subpoena you to appear before the grand jury, and then charge you with perjury. Once charged, you would have to get a lawyer, and you would have all sorts of other problems - like getting a job, bank loans, etc. Garrison would often dismiss the charges right before trial.
And he had the power to subpoena you with just the stroke of a pen. He just had to sign a piece of paper. Here is a document from the early part of the Garrison investigation.
While in office, the law changed an Garrison could subpoena you just to appear in his office for questioning. Not surprisingly, people were fearful of his power. Check out this headline.
As I detail in Chapter 1 - The Taking of New Orleans 1-2-3, of my book On The Trail of Delusion, Garrison discovered and used other powers. He bragged to Richard Billings, an editor of Life Magazine:
"I won't take no for an answer. I've not gonna let somebody lie to me in this case; they lied to the Warren Commission, they lied to the FBI, they're not gonna lie to me. I'm gonna use every legal form of power I have at my disposal. I have the right to made the decision. I have the power available, and I'm gonna use it. If they don't like the way I use the power, then the solution is for them not to elect me next time. But until the next election comes up, I have it and I'm going to use it as effectively as possible; and I'm going to use it in new ways, perhaps ways that haven't been used before, and with imagination and if neccessary with aggressiveness and audacity in order to get people who have concealed the truth previously, to tell the truth."
I have a lot more about how Garrison yielded his power in my book.