Zimmerman was charged by information, so there was no grand jury -- just an affidavit from the special prosecutor.
There is no defense - affirmative or otherwise - in grand jury proceedings. The state's attorney presents evidence to establish whether or not probable cause exists to charge a person with a crime. It's not uncommon for a defense attorney to present evidence to a state's attorney to submit to the grand jury but generally only the state's version of events is heard. Of course, returning indictments is not the only function of a grand jury.
Duties of grand jury.
540.031. A grand jury may make inquiry into and return indictments for all grades of crimes and shall make inquiry into all possible violations of the criminal laws as the court may direct. The grand jury may examine public buildings and report on their conditions.
In this instance, the DA's comments seem to indicate this is an " inquiry into all possible violations of the criminal laws," as the evidence is being presented as the investigation unfolds.
A grand jury's power is fairly broad:
540.240. All grand juries are hereby authorized to find and present bills of indictment for either felonies or misdemeanors committed against the laws of the state.
A grand jury is made of of 12 voting members and alternates which can serve for 6 months + 60 days. The convening judge has the power to dismiss the jury.
Selection of grand jurors, summons and jury qualification form--notification of persons not qualified to serve--alternate grand jurors--length of service--compensation
4. Those persons summoned for grand jury service shall be placed under the control and supervision of the presiding judge of the circuit court, or a judge designated by the presiding judge, who shall select twelve persons to serve as grand jurors. Alternate grand jurors as determined by the judge shall also be selected, to serve as a grand juror upon the death, disqualification, or inability of one of the persons selected as a regular grand juror. The names of those persons selected as grand jurors and alternate grand jurors shall be deleted from the grand jury list.
5. The presiding judge of the circuit court, or a judge designated by the presiding judge, shall have the authority to convene, recess, and adjourn a grand jury as, in his discretion, he deems necessary, and at times and places as he specifies. No grand jury shall be required to serve for longer than a six-month period, except such term may be extended for a period not to exceed sixty days, solely for the purpose of considering and completing matters already before the grand jury. No new matters shall be presented to the grand jury during its extended service. Nothing contained in this section prevents the convening of another grand jury during such extended service.
An indictment requires a supermajority vote.
True bill--concurrence by nine grand jurors.
540.250. No indictment can be found without the concurrence of at least nine grand jurors. When so found, and not otherwise, the foreperson of the grand jury shall certify under his hand that such indictment is a true bill, by the following endorsement thereon, thus: "A true bill. A B, foreperson".
Grand jury proceedings are secret.
Grand juror not to disclose evidence--penalty.
540.320. No grand juror shall disclose any evidence given before the grand jury, nor the name of any witness who appeared before them, except when lawfully required to testify as a witness in relation thereto; nor shall he disclose the fact of any indictment having been found against any person for a felony, not in actual confinement, until the defendant shall have been arrested thereon. Any juror violating the provisions of this section shall be deemed guilty of a class A misdemeanor.