What does the NDA answer?
Read the agreement for who may receive the material, permitted uses, disclosure limits, and any required return or deletion. Those contractual terms are not a substitute for the separate requirements of privilege. Keep public prior-art documents distinct from counsel's confidential legal analysis.
What does common interest add?
As the Ninth Circuit explains in In re Pacific Pictures, common interest is an exception to ordinary waiver rules, not a separate privilege. It concerns communications pursuing a joint legal strategy; merely wanting the same result is insufficient. That is a circuit-specific example, so counsel must identify the law governing the actual exchange.
What should happen before sharing?
Have counsel identify the underlying privilege or work-product protection, the legal purpose, recipients, and applicable waiver rules. Upjohn v. United States distinguishes confidential legal communications from underlying facts; Rule 26(b)(3) separately addresses trial-preparation material. Labels, copying lawyers, and signing an NDA do not by themselves establish that all exchanged material is protected.