Dear 100 Hour Board,
Spoilers for The Invisible Guest (2016)
At the end of the movie The Invisible Guest (originally in Spanish) from 2016, we are left with the truth about what happened to Daniel Garrido, but we don't find out if the man character gets arrested. Our main character has committed two murders and believes he is talking to his defense. Turns out he's talking to the mother of his first victim, who is secretly recording everything. If these events were to happen in the US, would that evidence be used to prosecute him, or would he be allowed to go free due to the way it was obtained? What about in Spain where the story takes place?
DISCLAIMER: I am not a lawyer, and this shouldn't be interpreted as legal advice.
It depends on the state. Federal law and several states only require one party in a conversation to consent to recording, but some states (California, Connecticut, Delaware, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, New Hampshire, Pennsylvania, and Washington, according to this site) require all parties to consent to a recording. Even if there are no issues with consent, whether the recording will be admissible in court will depend on if the prosecution can prove that the voice on the tape actually belongs to the person they claim it does, that it provides an accurate and contextual representation of the conversation, and that it hasn't been altered or tampered with in any way.
In Spain, only one participant in a conversation needs to consent to recording a conversation, according to this Spanish lawyer. However, data protection laws seem to consider voice recordings as subject to privacy laws, so I'm really not sure whether this case would apply. According to this site, the only exemptions to data protection laws are recordings made exclusively for personal or domestic use, to investigate terrorism and serious organized crime, and when protection of classified material applies. In my non-lawyer opinion, that doesn't include a secret recording as evidence in a murder case, but I don't know for sure.