In California (many other states are more liberal), you should always get the consent of all parties before recording any conversation that a reasonable person would find "private" or "confidential." Not to do so, exposes you to possible criminal prosecution, or more probably, a lawsuit for damages.
Conversations that occur in a "public" or in an area where the parties do not have any "expectation of privacy" are not covered by the unanimous consent requirement. You may record a conversation amongst people in a public place, such as a street or park, even without consent. For example, a California court has upheld a television network's right to use a hidden camera to videotape a conversation that took place during a business lunch on an outdoor patio of a public restaurant.
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