1. If your client wishes to testify before the grand jury, you must send the prosecutor a written notice of your client’s intent to testify. (CPL 190.50[5][a]). If the prosecutor is presenting on short notice, do your notice by email and fax, with an explanation included that you cannot send the letter by U.S. mail because of the short notice you received.
Some attorneys send these notices on each case both in order to preserve the client’s right and to preserve any potential issue of the prosecutor presents the case without providing the defendant with the opportunity to testify. But if you choose to engage in this practice, you must notify the prosecutor if your client is not testifying before the grand jury, as your client will be body-ordered to the grand jury, and may be left sitting, without you there, initially bewildered and eventually angry.
2. Often, it’s a bad idea for your client to testify before the grand jury. You don’t yet know the evidence the prosecutor has in the case, you don’t yet know whether your client’s version varies greatly with that evidence or with any statement s/he may have made to the police, the client’s testimony can be used at trial, the grand jury is likely to indict any case the prosecutor presents to them, and you may be revealing more than you should to the prosecutor prior to trial, allowing the prosecutor to try to prepare the witnesses to refute your defense. There are exceptions to the bad idea rule, but not many. (I should note that some of my colleagues engage in a more vigorous and less conservative grand jury practice than I do, with greater success.)