Write down information you are likely to need again: the objective, your structure, key numbers, assumptions, and the main conclusions from each section. You do not need a transcript of everything the interviewer says.

Keep the page organized so you know where to look. A simple structure page plus a calculation area is often enough. After a calculation, write the implication next to the number rather than only the raw result.

If note-taking is slowing you down, practice cases with an explicit rule that you only write facts that affect a future decision. If useful, this guide on case interview structure and flow gives more detail.