As a threshold matter, I don’t think immunity requires an agreement of any kind, because a witness can be immunized against his will and then compelled to testify, if he keeps resisting. The grant of immunity presumably should be documented, however.
But immunity is something that a prosecutor offers in order to secure a witness’s testimony against some other person in some criminal proceeding. That’s the only reason why a prosecutor would need to do so. Neither Castor nor Cosby’s defense team is saying that Castor gave Cosby immunity to obtain his testimony. Rather, the claim seems to be that Castor intended for his decision not to prosecute to constitute immunity – for no reason. Castor has nothing to do with the later civil suit. Rather, the fact that Cosby later testified in the separate civil case is only supposed to be evidence that he understood Castor’s decision not to prosecute to constitute immunity, because otherwise he would not have testified.
I am not a criminal attorney, but this all sounds like nonsense. Castor could have entered into a binding agreement with Cosby, but there is simply no reason to believe he did so, because he would have gotten nothing in return. Castor himself denies doing so. To all appearances, all Castor did was close his file. Jeopardy has not attached, and any subsequent prosecutor can come along, reopen the case, and file charges over the exact same conduct based on the exact same evidence.
Castor argues that his mere decision not to charge Cosby, in and of itself, had binding effect, but no sensible court is going to buy into that contention, because it would cripple the criminal justice system. Prosecutors make decisions not to charge all the time; indeed, the vast majority of what they investigate does not get charged. There can be all sorts of reasons why not: the evidence is weak, a key witness is absent, the harm appears minimal, or the staff is overwhelmed with bigger cases. But these cases always remain open to being revisited if the evidence improves (say, from a confession at a later civil deposition), the harm worsens, or the caseload lightens. Indeed, some network made a whole TV series out of cold cases. A court ruling that a mere decision not to prosecute forbids a prosecutor from ever reopening that file means that they not only have only one chance to try a person, they also only have one chance even just to investigate, and that’s too damaging to the whole process.
I don’t think Cosby’s supposed reliance upon his belief in immunity when he testified in the civil suit will amount to much, since it doesn’t seem at all reasonable to rely on a simple decision not to prosecute. Further, his current counsel has produced not only no evidence of an immunity agreement, but no direct evidence of Cosby’s belief in immunity back then. They seem to have found no correspondence with the civil plaintiff’s counsel about Cosby’s intentions as to his Fifth Amendment rights, and apparently no record in Cosby’s civil attorney’s files of any Fifth Amendment analysis or of him being counseled about immunity.
Not knowing anything about this judge, I may well be proven wrong, but I don’t see how Cosby has a leg to stand on here. I think the deposition testimony comes in.