Can an American material witness be forced to &quot;learn&quot; what s/he does not know? My understanding is that a witness is supposed to prepare for a deposition. My first concern is how well prepared or knowledgeable for a deposition. Suppose a material witness made a point of studying his adversary's liabilities thoroughly, and his own side's superficially. The witness then recites chapter and verse about his adversary's liability, and honestly says, "I don't know" about his side's. Can the opposing lawyer ask him, with effect, "why do you know so much about some topics, and so little about others?" Will the witness have fallen short in his duty? Can s/he be forced to learn more to be more responsive to the opposing lawyer's questions?
A witness can only testify to what they witnessed That means their testimony can only be what they personally saw or heard (or, more rarely, touched, smelled and tasted). They cannot testify to the truth of what someone else told them (although they can testify that they were told it) - that’s called hearsay and it’s inadmissible. A witness cannot learn things after the event and testify to them.
No, a defendant may not remain silent on cross-examination. Witnesses who voluntarily testify in their own defense are subject to cross-examination on that testimony. In Fitzpatrick v. United States, 178 U.S. 304, (1900), a murder defendant testified that he was at two bars and then his cabin the night of the crime. The trial court held that having waived his Fifth Amendment right to remain silent, the defendant was subject to cross examination about what he was wearing that night, his connections to a co-defendant, the co-defendant's clothes, and who else was at the cabin with him. The Supreme Court affirmed the conviction, holding that if a defendant voluntary makes a statement about the crime at trial, the prosecution may cross-examine him with as much latitude as it would have with any other witness: The witness having sworn to an alibi, it was perfectly competent for the government to cross-examine him as to every fact which had a bearing upon his whereabouts upon the night of the murder, and as to what he did and the persons with whom he associated that night. Indeed, we know of no reason why an accused person, who takes the stand as a witness, should not be subject to cross-examination as other witnesses are. Fitzpatrick v. United States, 178 U.S. 304, 315 (1900).
Signing as a witness binds you to nothing. Do not sign if you do not qualify as a witness (e.g. Time known, occupation) but otherwise go for it. It is unlikely that anyone would check your bona-fifes but if they did a statutory declaration would probably be satisfactory. Remember, the point of witnessing is to show the form was actually signed by the person.
In U.S. practice, double jeopardy protections attach once the jury is seated and this scenario contemplates the change in position arising at trial. A mistrial could be called, or a continuance obtained, if the defendant suddenly claimed an alibi defense, since that amounts of a defendant waiver of double jeopardy protections. But without proof that the witness was tampered with, obtained at the last moment, it can be difficult or impossible to get proof of tampering or to contradict the story with other evidence to undermine the credibility of the witness. The prosecution would probably request a recess in the trial to regroup and try to admit evidence that the testimony changing witness made a prior inconsistent statement to impeach the testimony of that witness, from whomever that witness told the first time (probably a law enforcement officer). But this is still far less convincing to a jury than an affirmative statement from a witnesses about a key fact. If the prosecution could find evidence of defense side tampering and present it to the judge in the hours or small number of days before the trial ends, it might be able to get a mistrial declared. But it is much harder to get a mistrial for witness tampering when the witness shows up and testifies contrary to prosecution expectations than it is when the witness is, for example, killed or kidnapped and doesn't appear to testify at trial at all. But if evidence of neither type could be obtained before the jury was sent to deliberate and evidence was closed, and the jury then acquitted the defendant, it would be very hard for the prosecution to change this result. Perhaps not completely impossible, but very nearly so. The witness could be (and in this circumstance, despite the extreme rarity of perjury prosecutions, probably would be) prosecuted for perjury, but that wouldn't change the acquittal. If a link to the defendant could be found, the defendant could also be prosecuted for witness tampering or obstruction of justice or something similar. But that is hard to prove (and the witness would probably have to be convicted or provided with use immunity for his testimony) to do so. In English criminal law practice, in contrast, the likelihood of having the verdict of acquittal set aside and getting a new trial would be much greater (and the likelihood of getting a recess mid-trial would be greater in far more common bench trials in lower level criminal cases), but it would still be a serious burden for the prosecution that it might not be able to overcome. Lawyers aren't entitled to expect that witnesses will testify exactly the way that they discussed things with the lawyers before trial, and this can be explained away with a variety of excuses that are usually true (&quot;after thinking it over after meeting with you, I realized that I was confused and getting it wrong&quot;), but are sometimes a cover for a change of story that is not sincere. I've had similar things happen a few times in civil litigation and there is only so much you can do about it.
Your question slightly misrepresents what the article says: Yes, the judge denied the motion which led to the collapse of the case, he did not make a ruling on the substance of the case. The distinction is significant to my mind as the judge was using non-evidentiary knowledge (i.e. what he read in the paper) to make a decision on process; in this case a process that would have put a lot of people to a lot of inconvenience. It would not be proper for the judge to have used such knowledge to inform a judgement. It is also not clear from the article if the academic paper in question was actually introduced by the defendant as evidence. If that was the case then it is only right and proper for the judge to consider it. As to why a judge is allowed to read the news and a jury is not, I can offer several ideas: A judge must document their reasoning process in a judgement which is subject to review - if they were to make a decision based on matters not supported by the evidence then an appeals court could correct it. Alternatively, juries are specifically prohibited from revealing their reasoning process to anyone. Judges do their jobs for years, perhaps a whole career - to prohibit them from consuming media is a) unworkable and b) a serious impediment on their lifestyle. Juries are empaneled for weeks or months - such sacrifices are more reasonable. Judges are (supposedly) trained and impartial professionals who are more readily able to make the distinction between evidence and news. Newsworthy cases are relatively rare
Misstating the truth is not perjury Perjury is deliberately lying under oath to gain a material advantage. For the situation you describe: You might be wrong and they actually do live where they say they do they might be wring and they genuinely think they live where they say they do, being wrong is not perjury it’s unlikely to administrate info in the form actually carries the penalty of perjury, it probably isn’t testimony unless it will positively and substantially affect the outcome of the case in their favour, it isn’t material You lack standing to interfere in the case in any event If this were brought to the attention of the court the most likely outcome would that it would just be corrected.
Parties may only call witnesses for the purpose of adducing* admissible evidence. Evidence is only admissible if it is relevant. If a witness cannot give any relevant evidence, then a party has no right to call them or invoke the court's power to compel them to give evidence. In practice, if there were any doubt about the witness's ability to give relevant evidence, it is likely that the court would allow the defendant to call the witness, if only for the purpose of a preliminary hearing where the parties can argue about whether the proposed evidence is actually admissible. If a judge or juror is able to give relevant evidence about a case, they should recuse themselves. If a judge does not recuse, or does not permit the defendant to call some other witness having decided that the witness could not give relevant evidence, these decisions can be reviewed on appeal. The standard required to overturn the judge's decision varies depending on the jurisdiction, but generally the defendant would have to show that the evidence they were not permitted to adduce was also material. This might prevent a mistrial from occurring in cases where, for example, the defendant was not permitted to call the President to give evidence about something that occurred at a public event the President attended. The President's knowledge of the event might be technically admissible, but plainly not likely to advance either side of the case, given the other evidence available. Of course, if the President can give relevant, admissible and material evidence about a fact in issue then the defendant would be entitled to call them. *Adduce: cite as evidence.
If you have something to say, you should have said it before now When a judge is about to hand down a decision the case is all but over. Just like figure skating at the Olympics, the points are scored even if nobody but the judges know what they are yet. The onus is on the parties to bring forward all the evidence and make all the submissions on the law that they want the judge to consider before and during the hearing. If they didn't then that's their fault and they can't introduce new stuff now. Now, it's not uncommon for a judge to share their thoughts during the hearing or in writing when considering written submissions. This is because their thinking on the law is at variance with what the parties are contending. The plaintiff says the law is X, the defendant says the law is Y, the judge thinks they're both idiots and the law is clearly Z. They will usually call for submissions on this because the judge's role is to decide the dispute between the parties on the evidence the contend -not to impose the judge's interpretation on them; by doing this the parties may relies the judge is right and a lot of the dispute disappears or they may convince the judge that they are wrong (it's not unknown) and move on from a clear agreed position.