A criminal defense attorney is arguably the most important person in your life after you've gotten arrested. While you might feel uncomfortable about sharing certain information with them, it's vital to share details regarding the following issues:
Past and Current Substance Abuse
You may be deeply embarrassed about a past alcohol problem or a casual drug habit that you had somewhere in your past. What's more, you may not even think it's remotely relevant to the crime you're accused of today, particularly if the charge has nothing to do with alcohol or illegal substances. For that reason, you might not want to suffer the humiliation of having to tell the lawyer about those issues. This reluctance to reveal information can be even greater if you are actively using these substances, but feel that they're not related to what's happening for you legally.
However, if you don't tell your defense lawyer, don't assume that the prosecutor won't track down that information themselves through witness interviews or other means. If the prosecutor should blindside your attorney in court with that information, your lawyer will have to scramble to come up with a defense against it. That could end up jeopardizing the outcome for you. Allow your lawyer to know all information about you, whether it embarrasses you or not, so that they can give you a strong defense.
Names of Possible Witnesses
Your defense attorney is probably going to ask you questions about others who might know about what happened to you. The list of people might include strangers on the scene, employees of shops in the vicinity, and your own relatives and friends. You might not want to involve certain people, so you may omit certain names from the list.
The primary reason that it's important to share names with your lawyer is to protect you. By notifying your lawyer about people that might provide insight into the situation, they might glean important details that you didn't realize those witnesses had. Not only that, but you no longer have to feel responsible for seeking out certain people yourself. In fact, it is preferable if you actively avoid anyone that could be even remotely related to the situation that got you arrested. Giving your attorney this knowledge will ensure that you have every defense tactic available to you without acting on information alone and making everything worse.
When you're forthcoming and open with your defense attorney, you can give them the tools they need to defend you. Communicate honestly with them to strengthen your legal case.