Special Counsel Robert Mueller is answering questions before two House committees tomorrow about his investigation into Russian interference in the 2016 election and alleged obstruction by President Trump. In his only other public comments on the investigation, Mueller said he would not go beyond the report in any testimony, but Democrats questioning Mueller tomorrow will surely be looking for more. Former Massachusetts Rep. Barney Frank spoke with WGBH News' Arun Rath to explain how Capitol Hill prepares for hearings like this. This transcript has been edited for clarity.

Arun Rath: You served in Congress for more than 30 years. I’m curious if you can talk about what members of Congress and their staff do to gear up.

Rep. Barney Frank: The first thing you have to decide is, what’s your relationship to that witness? There are friendly witnesses, there are hostile witnesses where you think you can frankly embarrass them, as I think I did with Ken Starr, by pointing out terrible inconsistencies in what they’ve said and done. There are hostile witnesses where you cannot get much done and your purpose is not to elicit information but to make clear to the watching public how wrong they are. So that’s the first decision, what kind of witness is this.

Rath: We’re hearing some people anticipate that Mueller might be not so much a hostile witness, but a reluctant witness.

Frank: That’s exactly right, and that’s what you would do. You would not try to embarrass him. He’s not done anything wrong. You would not try to contradict him. What you should do is to look at what he has said. Yes, he’s a reluctant witness, but he’s also one of the most intellectually honest people to ever get into politics. That’s sometimes frustrating for some of us, and I think he overdoes it, but you would look at what he has set down as his own limitations, and within those limits ask him questions that will lead him to elaborate on what he thinks, as he’s not going to repudiate some of the red lines, where he said he wouldn’t talk about it.

But I think you can try to get him to explain the reasoning — I guess this is what I would do — I would try to get him to explain the reasoning and the evidence behind some of the conclusory statements. I wouldn’t challenge his conclusions, but when those conclusions were in many cases implicitly and occasionally explicitly critical of the president, as a Democrat, I would want him to elaborate on the implications of those conclusions.

Rath: Obviously this is a very hyper-partisan time that we’re going through right now, but this is a political process even in the best of times. Can you talk about how you would advise your party to approach this politically, as well as what their responsibilities are?

Frank: The way to do it is to be fairly straightforward with a straightforward man. Think about the most negative implications of what he said for the president and try to draw those out.

Rath: How important a moment do you think this is, overall, for the politics of dealing with President Trump?

Frank: It’s not very important. In this particular case, with the lines drawn in the country with a group of people, I believe a minority, ardently pro-Trump and then a number of people critical, I don’t think this is going to change many minds. I would hope in fact that overheated expectations don’t lead it to be misinterpreted as Trump getting off the hook. I think it is an opportunity for the Democrats to spell out and elaborate on the implications. There are very negative implications in what the Mueller Report said, particularly about obstruction. I wouldn’t go into the collusion much, he said he didn’t find any. I would go into the obstruction and talk about his conclusion that there was obstruction, and implicitly he couldn’t go after it because you can’t indict the president, and I would have him spell that out.

Rath: Big picture, regardless of whether this testimony changes any minds or has a big effect or not, where do you think we are? Are we moving towards impeachment?

Frank: I hope not. I hope not. I would hope that we would be moving towards a debate on his terribly unfair tax bill and the enormous problems it’s helped create with the deficit, that we would talk about the absence of infrastructure. No, I do not think a partisan vote in the House to impeach the president, which is then repudiated by a partisan vote in the Senate gets us anywhere. I believe there are strong reasons to be making the case against Trump, but I think at this point, the kind of impeachable offenses, the abuses of power, those are given, people know what they think. I think what we as Democrats ought to do is to stress and get more attention to those terrible gaps in policy, and in particular his absolute failure to deliver for the people he claims to be championing. I would hope that would get the attention.

Rath: Congressman, thank you very much.

Frank: You’re welcome.

Rath: That is former Massachusetts congressman Barney Frank, giving us some insight into Special Counsel Robert Mueller’s scheduled testimony before the House Judiciary Committee and the House Intelligence Committee. WGBH News will be carrying it live tomorrow morning starting at 8:30am on 89.7 WGBH and on wgbhnews.org.