What the agreement protects and what it does not
Sharing privileged information with an outsider ordinarily waives the privilege. A joint defense agreement, sometimes called a common interest arrangement, is the framework lawyers rely on to argue that communications among parties with a shared legal interest remain protected against everyone else. New York's highest court has read that protection narrowly under state law, generally tying it to pending or reasonably anticipated litigation, while some federal courts have taken a broader view. The shared interest has to be legal, not merely commercial. The agreement does not make one party's lawyer the lawyer for the others, and it does not stop a member from testifying about facts they personally know.
When one member starts cooperating
The most common point of stress is a member who decides to cooperate with prosecutors or settle separately. A carefully drafted agreement requires notice of withdrawal, addresses what happens to materials already exchanged, and usually includes a waiver so that counsel for the remaining members can cross-examine a former member who becomes a witness without facing disqualification. Even with those terms, cooperation can expose what was discussed, so many lawyers limit what they share and how. Agreements can be oral, but a written one reduces later arguments about whether an agreement existed and what it covered. These terms are often negotiated in a hurry, which is when mistakes get made.
Deciding whether to join
Joining is not automatic. Interests that look aligned at the outset can diverge, and an agreement may make it harder to change strategy later. Before advising a client to sign, we look at who the other parties are, how their exposure differs, what the government or opposing side already knows, and whether anything shared could later be turned against the client. For individuals, a separate question is whether a company paying legal fees is also part of the arrangement. We review the draft line by line and explain what each provision gives and what it takes away.
