Materialization out of thin air means that the succubus appears before you out of nowhere as a physical person. She looks human but in your interactions, she is referring to herself as a succubus. You can discuss who she is, her world etc with full clarity.
Acting as human means that the succubus appears in a place where noone can see her. She plays the role of a human, as if acting on stage. While you may be aware who she is, she will give you no clear sign that she is a succubus. She may give you hints, but in all your interactions, she is following the set role that she picked. For example she plays the role of an exchange student, you can bring her to meet your friends/family and everyone will go by the role of her being human.
Switching to my perspective, why do I care about this?
I read about someone who had a succubus materialize right before him, and would want to know how this is achieved. It's not a matter of a limitation from the side of the succubus. To be honest, I have "mine" around me all day. This is all fine as they are acting a role and filling in things that humans could have done, but have slacked off with or refused to do.
When I met a new succubus who I was expecting to work with for a while, I asked her to appear to me physically so I could get an idea of what she looks like. I knew she would agree, but not how or when she would come. A few days later I met a customer and realized it was her. This way I got an interaction with her, hearing her voice and so on, it was what I had asked for.
By telling this story I want to make clear that the manifestation itself is not the problem. The obstacle is the human mind. I've seen a succubus appear/disappear on an empty street about 100 meters away from me. This was someone who was acting a role, so she told me her family would pick her up, called someone on the phone, walked a bit and was gone. So it seems this much is ok.
But to really have someone appear directly, with your full awareness of the situation - from my understanding this is not possible if your mind can't accept it. Even if you know how things work, what they are, and so on, your mind can still be too narrow to take this experience. You would get scared, freak out and it would turn into a negative experience. Maybe it would make any further interactions impossible. Similar to if you really saw a ghost. You may say it wouldn't scare you, but what if someone walks in on you while you are on your computer, have headphones or something, and they make you jump because you think you where alone? Then it may still be a person you know and you react with getting scared. So what if it is a stranger? You're not going to take that lightly, if it was in your own home.
The idea may be hard to grasp. But say if you invited someone to come to your place, you made certain preparations. Maybe you told other people about the visit, you prepared something to eat or drink, a movie to watch or a game to play. When doing this you are trying to sense what the person likes and adapt. Can you imagine doing all of this to prepare for a succubus? You can't tell someone about it either, so that connection can't exist. Everyone will think you are alone, but you need to be in a social mindset.
Did your succubus materialize out of thin air? How did you make this happen, all steps you performed to get there?