Joyce Vance: You Don't Use A Big Fish To Catch A Little Fish

Former U.S. attorney Joyce Vance gave a short master class on Maddow's show Friday, with Ali Velshi hosting.

"You know, there are different flavors of immunity and they come with different rights and responsibilities. but one of the important things to remember about immunity is that it's always better to have a witness cooperating with you voluntarily because they'll share information that they know," she said.

"They're doing that to get a good deal for themselves. When you give a witness immunity, you compel them to speak to you. The only answers you get are the answers to the questions that you know to ask that witness. So it's always preferable to have someone cooperating voluntarily and it's just not clear yet which what is going on here," she said.

Velshi asked about the fact that, when Michael Cohen's guilty plea was announced, -- "Strangely, the prosecution, the Southern District of New York, did not announce a cooperation deal with him. Now a number of lawyers -- and you and I have spoken, you have told me that sometimes a cooperation deal emanates from a plea deal. Ehere do you think we are on that?"

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