WASHINGTON (CNN) -- A face familiar to perhaps most of the American public may take the witness stand Monday in the criminal trial of Lewis "Scooter" Libby, a former top aide to Vice President Cheney.

Ari Fleischer, former press secretary to President Bush, is the next prosecution witness in the case of perjury and obstruction of justice, and the first to testify under a grant of immunity. He was at the courthouse Thursday and told reporters on-camera that he plans to "tell what he knows."

He did not elaborate and departed without being called to the stand.

After the jury was excused for the week, defense and prosecutors argued Thursday before the judge about Fleischer's upcoming appearance. Special Counsel Patrick Fitzgerald, fighting a defense request to know the basis for Fleischer's immunity, told the judge "I did not know what we were going to get when he went into the Grand Jury, other than I knew it was going to be relevant to the case."

In court papers filed over the weekend, defense lawyers said they have the legal right to know what led up to a deal for leniency, so that the jury can evaluate the motive and credibility of the witness.

The prosecution argued that there is no formal "statement of facts" from Fleischer to provide in response, but the defense cited case law to say a tacit agreement for testimony still obligates disclosure of information or evidence that could be helpful to the defense.

Prosecutors are trying to portray Libby as someone who used lies and deception in his ranking position at the White House to try to discredit an open critic of the Bush prewar justification to invade Iraq.