So a police officer or federal agent shows up at your home or office and says that he or she wants to ask you some questions. They tell you that you're not in trouble but that they're investigating a situation and they want to see if you can shed some light on it.
For a number of reasons, you're going to find it very difficult to not answer their questions. You're sure you didn't do anything wrong and feel that you have nothing to hide. The presence of the police is intimidating. Even people who know they've got 50 pounds of weed in the trunk of the car will talk to the cop who pulled them over and end up saying something that gets them busted.
Resist the temptation. Resist that feeling you have that if you don't answer their questions they might think you're guilty. Don't do it. You have a constitutional right to stay silent. Exercise that right. "I'm sorry officer, but I'm not inclined to answer any questions."
Let's look at a real world example. A woman you met recently invites you over to her apartment to hang out. You both have a few drinks and you end up spending the night and having sex. A few weeks later your date goes to the police and says that you raped her. By that time there is no physical evidence – it's too late to do a rape kit. The only thing the police have is what the woman is telling them. So the police show up at your door to ask you a few questions. At that point they don't even have evidence that you had sex with the woman, much less that you raped her, other than that the woman said so. Beyond what she's saying, they have no proof at all.
The cop asks you to tell him about your date, so you do. 'I went over to her place, we hung out, we had some drinks, we had sex, I haven't spoken to her since." the cop tells you that she's saying you raped her. You insist that you didn't rape her but that the sex was consensual.
What have you just done? You've corroborated everything the woman said to the police except the issue of whether the sex was consensual. Before they talked to you, they couldn't even prove you had been to her apartment, or that you had sex with her. Now that you've admitted most of what she said, the only issue left is whether the sex was consensual. So even if you are innocent and only tell the truth, you will almost always give the police some information that can be used to convict you. Exercise your constitutional right. Don't answer their questions.
So what about when they ask you if they can search your house or your car. Say no! It doesn't matter how sure you are that you've got nothing in the car to be worried about. You could end up sitting on the side of the freeway for an hour while they pull up your seats and dismantle the door panels. Why would you let them do that? The cop may tell you that you'll have to stand by until he can call a canine to the scene if you don't consent. Let him call the canine. If the cop searches your car without a warrant or without probable cause and finds something, you might be able to get the evidence tossed through a motion to suppress. If you give consent to search it's pretty much all over. Don't do it.
One last thing, in case you're thinking, "He's a criminal defense lawyer. Of course that's what he'd say." These are the words of Justice Robert Jackson who was a career prosecutor and Attorney General of the United States before he was named to the Supreme Court:
Any lawyer worth his salt will tell his client in no uncertain terms to make no statement to the police under any circumstances.
I'd like to think I'm worth my salt, and that's what im telling you too.