Democrats are pushing to have former White House National Security Adviser John Bolton testify in the U.S. Senate's impeachment trial of President Donald Trump. WGBH News Morning Edition host Joe Mathieu spoke with Northeastern University law professor and WGBH News legal analyst Daniel Medwed about the value of live witness testimony, and how it could play out in the impeachment trial. The transcript below has been edited for clarity.

Joe Mathieu: Some senators, certainly the president's own legal team, say it doesn't matter what a witness would say. It won't change a thing here. Does it matter, the value of live witness testimony versus documents or other testimony we've heard?

Daniel Medwed: Well, I think it does matter, and here's why. Most notably, it is direct evidence firsthand evidence in contrast to secondhand hearsay testimony, for instance. Also, it's live. The jurors in this case, the Senate would get to evaluate the demeanor of the witness in real time in the witness box. And that's a significant advantage. Also, there are other protections to ensure reliability. You have to testify under oath, under penalties of perjury, there's cross-examination, and lawyers have what are called witness impeachment tools available to go after the witnesses.

Mathieu: That's different than impeaching the president, I presume.

Medwed: Yes, that's very important. When I talk about witness impeachment in the generic sense, it's different from the constitutional concept of impeachment in terms of removing a federal officeholder from a position of power. Rather, it refers to the technique of undermining or discrediting a witness on the stand.

There are two broad categories of impeachment. The first is non-character based. It's when you try to say that a witness is not someone who is a liar, who has a poor character for telling the truth, but they have some reason some incentive for lying now. Classic example: witness bias. You point out that a witness has a financial stake in the case or is related to one of the parties or has a longstanding ax to grind with someone affiliated with the case. The other technique is, in fact, character based. That's when you introduce evidence that suggests this person is a liar. They have a poor character for telling the truth.

Mathieu: I can only imagine where that would go with Bolton potentially on the stand. More on the character of a witness. Daniel, as a practical matter, how would you get at that?

Medwed: Well, one very common technique occurs after the witness has testified and you put on your own witness to offer what's called opinion or reputation testimony about truth or veracity. You ask this witness, 'In your opinion, based on your experiences with John Bolton, is he to be trusted?' or, 'What is John Bolton's reputation for honesty in the relevant community?'

Another technique that can be very effective is to simply cross-examine the witness. If you have a good faith basis for believing that the witness has done something in his or her past that reflects dishonesty, you can ask about it. 'Isn't it true that you lied on your law school application or you plagiarized in high school?' Something like that.

Mathieu: So in a courtroom, at least, you can ask witnesses about past acts that go to honesty. If there's a criminal record, for instance, can you ask about past crimes as well?

Medwed: Now, that's a tricky one. On the one hand, the general answer is yes. A witness's criminal record is fair game for impeachment based on the rationale that if you've committed a crime, you've transgressed against society's norms [and] you've violated legal standards. And that, to some extent, is indicative of dishonesty. But on the other hand, especially with criminal defendants, we're wary of dissuading them from testifying, basically pushing them to invoke their Fifth Amendment privilege against self-incrimination for fear that they'll be impeached with their criminal record. So for that reason, the rules are stricter [and] more protective of criminal defendants when allowed, when it goes to impeaching them with their criminal record. At least that's the theory. In practice, Judges are pretty willing to impeach defendants.

Mathieu: We're talking about life in a courtroom. That's where you're from. Does all of this mean the same in the Senate?

Medwed: That's what's been so baffling about watching this impeachment trial unfold, because it really doesn't feel like a trial. A trial is when you present witnesses [and] vet the evidence. It's an adversarial testing of the facts. Here, it's more like a high school debate, with each side raising their arguments in a vacuum. So I think you're right. I can only speculate about whether these courtroom techniques will eventually translate into the well of the Senate.

More:

What Live Witness Testimony Might Look Like In Trump's Impeachment Trial - wgbh.org