Nobody really wants to end up on the witness stand in a federal criminal case. The odds are good that—if it has happened to you—you're only there because you've been caught up in the circumstances of the case and are under the threat of charges yourself.

Here are some important things to know.

It's very easy to get caught up in a federal criminal investigation.

Federal laws regarding things like money laundering, racketeering, insurance fraud, and the like are so complicated that people can end up guilty of a criminal act without even realizing what they've done. When the federal investigation comes tumbling down around them, the prosecutor really isn't interested in these low-level players. Instead, the prosecutor will use the carrot-and-stick method of getting these people to become witnesses for the prosecution.

• What's the carrot? The district attorney can offer immunity from prosecution.
• What's the stick? If you don't take the district attorney's offer, you get prosecuted as harshly as possible, regardless of your actual involvement in the alleged crime.

If you're in this situation, however, it's very important to understand exactly what kind of immunity you're being offered before you accept the deal to testify.

There are two distinct kinds of immunity that you can be offered.

You may be offered transactional immunity in exchange for your testimony against other defendants. This is the broadest, most-valuable type of immunity you can be offered. It is blanket protection against prosecution for anything that you might say in your immunized testimony.

For example, if you are testifying in a Medicare fraud case, you can admit, in open court, that you knew the doctor you worked for was illegally billing for services he never performed. You can admit to filing the paperwork for him because you didn't want to lose your job. While you're admitting to a crime, you can't be prosecuted for it if you have transactional immunity.

You may also be offered "use" immunity. This prevents the prosecution from using your testimony against you later to prosecute you–but it doesn't stop the prosecutor from gathering evidence regarding anything you admit on the stand and then using that evidence to charge you with a crime.

For example, if you admit under use immunity to knowingly filing the illegal Medicare paperwork, your testimony is protected from use. However, the prosecutor can go back through the evidence and try to put together enough independent proof of your crime to convict you of the fraudulent billings anyhow.

If you're a witness in a federal criminal trial, make no mistake–you need your own federal criminal lawyer to help you understand all of your options and liabilities before you make a deal with the prosecutor or step into court.