After a first interview the note should be short and reinforce one point of fit. You are one of many candidates at this stage — a specific reference to the conversation is what separates your note from the templated ones.
Within 24 hours, while the interviewer still remembers the specifics of your conversation.
Email, almost always. It arrives while the decision is live. A printed letter is a nice supplement for traditional industries, never a replacement.
Send each of them a separate note referencing what they personally asked about. Never send the same text to everyone on a panel.