In preparing for a deposition, it's easy to slip into this mindset: anything that's relevant to the case is already contained in the file. Review the file and that's enough.
It's lazy thinking. In these days when information is cheap, it always makes sense to query Google about your witness. When you're done there, do a search in a comprehensive news database like those found at Westlaw or Lexis. You'll never know what you'll turn up.
Often, witnesses have their own websites. This is especially true of experts. When reading the expert's website, look for mistakes, exaggerations, and anything that might contradict the testimony the witness is expected to give at trial.