The common phrase uttered by prosecutors asked to grant immunity (or even enter into a plea deal in exchange for cooperation) is "We won’t buy a ‘pig in a poke.’" Having uttered it myself with no clue what it meant, I once looked up the phrase and learned that in an earlier era pigs (and other animals) being transported to a market to be sold were kept in a "poke" sack, which is a thick canvas bag, to keep them from escaping — pigs were especially prone to running away, so the sack was kept tied tightly. Rather than buying something sight unseen (like we do on the internet all the time), the purchaser demanded to see the animal before paying for it, lest he buy the pig in an unopened poke. For prosecutors, the phrase is an indication that the witness/defendant has to provide a statement, usually called a proffer, before the government will make a deal.
The prosecution of I. Lewis Libby has brought this timeworn phrase to the forefront with news that former White House spokesman Ari Fleischer sold the aforesaid pig in a poke to Special Counsel Patrick Fitzgerald when he demanded immunity during the investigation of the leak of Valerie Plame’s status as a CIA agent without first making a proffer. Fleischer is scheduled to testify against Libby at trial, and the prosecutor refuses to turn over any notes because they are not witness statements (under the Jencks Act) nor was there a deal for Fleischer to receive immunity in exchange for certain testimony. In that sense, while the government knows the outlines of Fleisher’s testimony, the exact nature of it will be a surprise to both sides, at least according to Fitzgerald. Needless to say, defense counsel are skeptical that an experienced prosecutor like Fitzgerald would make a blind immunity deal and are demanding to know what has been said about Fleisher’s testimony.
An AP story (here) quotes Fitzgerald as stating in court, "I didn’t want to give him immunity. I did so reluctantly . . . I was buying a pig in a poke." Although I doubt Fitzgerald meant the porcine reference to relate specifically to Fleischer, it certainly shows that the Special Counsel was willing to take a significant risk that Fleischer would say things that would protect others in the administration while making it virtually impossible to pursue criminal charges against him because of the immunity. While risky, the grant of immunity without hearing Fleisher’s likely testimony allows him to state on the witness stand that he did not "bargain" for that protection in exchange for "tainted" testimony, thus giving his credibility a small but potentially important boost. Never having bought a real pig in a poke, I can’t say what the risk there really is, but in a high-stakes investigation like this one Fitzgerald has shown his willingness to take chances. To the extent the prosecutor denies having any information on what Fleischer promised to testify about, U.S. District Judge Walton’s hands are largely tied because he can’t order the government to turn over what it doesn’t have, even if the government purposely chose to avoid creating such material to hamstring the defense. (ph)