Q: When publishing a true crime novel, how can an author protect themselves from lawsuit?

A: You should ensure that all proper copyright registrations are in place, obtain a book clearance report, general liability and errors and omissions (E&O) insurance, and proper contracts with publishers and other parties. “Script clearance” means that a clearance service will go through the script and create a report that flags potential legal problems relating to copyright, trademark, defamation, right to publicity and privacy issues. "General Liability Insurance" covers a number of liabilities, including, defamation and copyright infringement claims against you. "Errors & Omissions Insurance" covers claims against you brought by a client, publisher, or a customer, where the other party claims you were negligent, failed to deliver the book you agreed to, or delivered shoddy, incomplete, or unsatisfactory work, etc.

A: Is it a novel or a real event?

If it's a novel, usually a disclaimer of any relation to fact is sufficient, assuming that it's not obvious.

For instance, real life characters appear in Forrest Gump, but that doesn't mean anyone thinks they are real or that the real person was as depicted.

As to non-fiction, the problem you get into is when you are engaging in conjecture. Is the subject alive or dead? After being sued, novelist Joseph Wambaugh said his advice is "only write about dead people."

However you may be putting things in reverse order. See if anyone will publish it. They will tell you what they want. If you are thinking of publishing it yourself then it's all on you.

If it's a novel and you feel you must stick to each and every fact, maybe fiction isn't for you?