At trial, the credibility of a witness is usually a matter for the finder of fact. The jury, or the judge in a bench trial, must weigh the credibility of each witness and decide what weight, if any, to accord to his testimony.
It’s possible for basic credibility to be a matter of law, as a threshhold determination – that is, a judge may decide that as a matter of law, a particular witness’ proffered testimony is incredible, and thus bar him from testifying.
The hostility of a witness is determined by the judge as an evidentiary ruling. The side wishing the witness declared hostile has to show the judge why the witness is hostile - if the witness’ interests are adverse to the side calling him, for example. Such determinations are reviewed on appeal under an abuse of discretion standard.
Testimony and other evidence stricken from the record may still be referenced on appeal if you want to show that an error was made in striking, but the evidence is not “part of the record” of the trial, and may not be considered on appeal.
For example, let’s say you’re accused of bank robbery. The robber wore a mask of Bill Clinton into the bank, and none of the witnesses in the bank could indentify the accused as the robber. However, the prosecution puts on a witness who says she was in the alley across the street from the bank, and she saw the accused run out of the bank wearing the mask, run across the street into the alley, remove the mask, and get into a car and drive away. A police officer testifies that based on the woman’s description of the car, and the license plate number she provided, the accused was identified by name, arrested, and brought to the station, where the witness picked him out of a lineup. He concedes that a search of the man’s home and car failed to reveal the stolen money, but several masks were discovered, including a Bill Clinton mask.
On defense, the alley witness is recalled to the stand, and asked if she was arrested for drug use. She admits that she was, and further admits that in fact was still in jail during the date and time that the robbery occurred, and tearfully admits she made the whole story up to get back at an ex-, who she knew happened to own a Clinton mask. Based on this admission, the judge orders her entire testimony stricken.
Now, normally at that point the judge would grant a defense motion for a directed verdict, because the prosecution has presented no evidence on the record that establishes the guilt of the accused. But, because the judge is in error, he lets the case go to the jury, who find the man guilty anyway.
On appeal, the appellate court will not consider the testimony of the woman at all. They will review “the record” and note that the only evidence adduced by the prosecution is that a search of a man’s home revealed a Bill Clinton mask. This, they will point out, is insufficient evidence upon which a finding of guilty may be sustained.
Of course, the prosecution can argue that the judge made an error in striking the testimony, and that the jury should have been left to decide the weight to be accorded to the woman’s story. To resolve that question, the appeals court will review the “stricken” testimony.