What is litigation privilege and what is the argument that litigation privilege means?

The oft-quoted explanation for litigation privilege is found in Hickman v. Taylor, (1947), 329 U.S. 495, 91 L.Ed. 451, page 511: In performing his various duties ... it is essential that a lawyer work with a certain degree of privacy, free from unnecessary intrusion by opposing parties and their counsel. Proper preparation of a client’s case demands that he assemble information, sift what he considers to be the relevant from the irrelevant facts, prepare his legal theories and plan his strategy without undue and needless interference.

In Breau v. Naddy, 1995 CanLII 3040 (PE SCTD), 133 Nfld. & P.E.I.R. 196, the policy basis for litigation privilege was described in this way: Litigation privilege is grounded in the proposition that counsel must be free to make the fullest investigation and research without risking disclosure of counsel's opinions, strategies or conclusions.