jno, it often happens that witnesses volunteer evidence in the box, which evidence has not been in any of their statements served by the prosecution. Defending, your first line is to ask why they hadn't mentioned it before; is it a recent fabrication ? You can ask for time to take instructions from the client so that you can put their version in cross-examination or so a witness can be found to deal with it, depending on what it is. In an extreme case, you may have to seek and adjournment or have the trial abandoned for retrial.
However, if the witness suddenly volunteers something so important as to make you consider the last courses, you'll probably think the first course is adequate. It is manifestly unlikely that the witness would not have given that, or been asked, when making statements for the court; it shouts malice and invention.