The life of the characters

The last weeks I was often asked if the characters I have built around a story I write at the moment are really fictive.
I always tried to explain that they ARE fictive and not existing in real life.
Ok, I explained it hundreds of times, but some people will not believe it.
I created a new person in the story and again, there was the comment, that this person sound so real, that it can´t be, that they are JUST fictive.
So, I thought a lot about that subject and came to a conclusion.

Yes, this people are not existing people, just fictive. But they are in my head now for the last few month and so they became kind of real to me. I mean, I lived with them and had conversations with them (only in my head) and so they have a life, kind of....
That may be the reason, why they sound like they are not fictive.
I think this is the meaning of writing, to be able to create a character so real, that people can imagine, that they really exist.

Do you have experiences like that and how do you explain, when people ask?