Below is a worked example for one state — California audio recording — laid out the way every situation needs reading: the plain-English rule, the actual statute, the traps that catch people, and the interstate wrinkle. Consent rules (one-party vs all-party) vary by state, so treat this as a model for the questions to ask and confirm your own state's rule before you act — start from our state-by-state PI guides. It orients you fast; it doesn't decide for you.
California requires everyone in a confidential conversation to consent before it's recorded — not just you. Recording a private call or in-person conversation without all parties' agreement can be a crime and grounds for a civil lawsuit. In practice: get clear consent on the record, or don't record.
It's one of a minority of all-party (often called "two-party") consent states — roughly a dozen, though sources differ on the exact count and a few states split the rule between phone and in-person. The other ~38 states plus D.C. follow one-party consent, where your own participation is enough. California is among the strictest, and notably enforces its rule against out-of-state callers who record Californians.
When a call crosses state lines between a one-party and an all-party state, there is no universal rule on which law wins — courts have variously applied the recorder's state, the recorded party's state, or the storage location. California specifically has applied its law when either party is in California (Kearney v. Salomon Smith Barney, 2006).
This is orientation, not a legal opinion. Recording law changes, has fact-specific exceptions, and varies by medium (phone vs. in-person). Read the linked statute and confirm with an attorney licensed in the relevant state before you record.