Theo chuckled. "Good, if you don't like it it's that much easier to push someone out," he answered. "That's why I wanted you to feel me come in."
He thought about the question for a while. "Some are," he then said. "Though it's very much a matter of both skill and preference. Occlemency is my strong suit, I know enough Legilimency to do this, but I've never been a brilliant Legilimens. Someone with say the Dark Lord's power could enter your mind and you'd never know he was there."
"Fortunately, most of the people who you want to block wouldn't know how to be subtle if their lives depended on it. The Dark Lord is..." he tried to think of an appropriate word. "dramatic. You sense him because he's not trying to make sure you don't. Bellatrix-" She was similar to his father. "I haven't experienced her personally, but from what I've heard and seen, she likes the power. She likes to break people and toy with them. You'd definitely know if she was in your mind. She's powerful though, and won't hesitate to use force to enter your mind. If she thinks you're hiding something she will torture you until you can't resist any longer." Now that was the real issue here.
He paused. "I wouldn't know about the rest, I doubt most of them have the patience for something this complicated."
"However, what you need is more complex than just pushing them out of your mind," he said. "It's a start, and I wanted you to do it so that you know how this feels like. But you need to learn how to shield, first your mind in general, and then the particular memories that you want to protect. You won't be safe until you can let them be in your mind while still hiding what you don't want them to know."
"Eventually you might be able to fabricate memories to replace the ones you hide, but that is more complicated still. We'll start with a shield, then work our way up. Either way, you need to get used to having someone in your mind, because if you kick them out, they're only going to get suspicious. Knowing how is good, it helps you when shielding, but in the end, it's not what you need."
He looked at Stephen again, and without warning entered his mind again.