Actor Kevin Spacey was arraigned last week on Nantucket on a charge of sexually assaulting an 18-year-old man. The case, which has garnered widespread media attention, will resume with a preliminary hearing scheduled for March 4. Morning Edition Host Joe Mathieu discussed the road ahead with Northeastern Law Professor and WGBH Legal Analyst Daniel Medwed. The transcript below has been edited for clarity.

Joe Mathieu: So first of all, if this case were to go to trial, what are some of the challenges facing the prosecution?

Daniel Medwed: Well, it's shaping up as a classic credibility contest between Spacey and the accuser. There's no physical evidence, which I wouldn't expect in a two-year-old groping case, and at least according to the defense, no witnesses have come out of the woodwork from that bar that night. But it's important to note that you don't need corroboration in a criminal prosecution. It's enough for the victim's testimony to prove guilt beyond a reasonable doubt, if the jury believes that testimony. So Spacey's defense right now, they're probably engaged in a scorched earth investigation to come up with information that could discredit the accuser's account.

Mathieu: So what might those tactics consist of? Can they actually attack him specifically?

Medwed: Well, yes. There is something called witness impeachment, and that's when you impeach or undermine the integrity and credibility of a witness on the stand. And there are two broad categories of witness impeachment. Non-character based impeachment is when you introduce evidence suggesting that the person is lying now — not that the person's a liar, but maybe there's a financial incentive, or a bias, or past inconsistent statements, or, as in this case, maybe intoxication somehow made it more difficult to recall what occurred. Then there's character-based impeachment, where you're introducing evidence that yes, this witness is a liar, because that's very salient to the jury in evaluating credibility. So the defense would put on evidence, opinion or reputation evidence, or maybe on cross-examination they'll ask about specific instances of conduct that bear upon the witness's character for truthfulness or untruthfulness.

Mathieu: Are there limitations on the prosecution here when attacking the witness? What can't the DA do?

Medwed: That's a very important question. Here's what they can't do. They can't ask about the accuser's past sexual behavior or sexual predisposition — that would violate something called the rape shield law. Massachusetts Rule of Evidence 412 says that in a sexual assault case, we need to protect alleged victims from having their past sexual history dragged through the mud.

Mathieu: We're talking with WGBH News Legal Analyst Daniel Medwed about the road ahead here in the Kevin Spacey case. Now I'm thinking about the Cosby case here, and I don't want to necessarily combine these two, but it's important as a point of reference. Daniel, may the prosecution introduce evidence of allegations made by other men against Spacey? Because we have heard there are others out there.

Medwed: That's right. I think there's something like 15 other accusations floating out there. That's a critically important question. And here's how it works. There's another Massachusetts rule of evidence 404b that governs the admissibility of what's called other act evidence like that. So on the one hand, you can't introduce all of this evidence of the other accusations simply to prove character propensity, that Kevin Spacey is a bad guy and he acted in accordance with that bad character trait on Nantucket Island. On the other hand, you may introduce those other accusations to prove something other than character propensity. For instance, in the Bill Cosby case in Pennsylvania, to show a method of operation, a modus operandi. Is there a very distinctive idiosyncratic pattern of behavior — sexually predatory behavior — that the jury should hear about? Now that line between character propensity evidence and M.O. evidence is very murky, and it depends on how idiosyncratic and specific the pattern of behavior is. At least, I don't know what that is. So far I don't know enough about the 15 other allegations, but we may know more in the months ahead.

Mathieu: And you're referring to Cosby's pattern of, for instance, drugging women and so forth, slipping something in the drink?

Medwed: Absolutely. With Bill Cosby, there was a lot of evidence of a very specific pattern of behavior, where he would ask women to his room, he would slip them drugs into alcoholic beverages, and an incident would occur. He would sexually assault them. It doesn't seem like we have enough information yet about any similar pattern of behavior with Kevin Spacey. That's not to say it didn't occur. We just don't know yet, because my understanding is none of those cases have resulted in litigation yet. And that's one of the reasons why the Nantucket case is garnering so much national attention.