You ask them "Ok, so what's your interest?" They tell you. You ask "Well, what aspects of that topic do you know about?" They tell you. You say, "Well, a person with AS might be obsessed with knowing *everything* about that topic if it was his or her special interest." Let me line up a scenario for you.
King Tut: People on the autistic spectrum are intensely interested in one topic.
Johnny Neurotypical: Man, everyone is like that. Everyone has something they're interested in more than everything else.
King Tut: Ok, so what's your interest?
Johnny Neurotypical: Well, I like to collect comic books. I'm also a huge fan of the Raiders.
King Tut: So, what do you know about comic books? How about them Raiders?
Johnny Neurotypical: I've got Superman comics dating back to 1959. I know all the characters names. And the Raiders, well, I know they've got three Superbowls, I know all their past coaches, and of course I'm following the current season.
King Tut: Well, let's say it like this. If you were an autistic, and your special interest was Superman comic books, you'd try to know everything about them. You might know the names of the artists, what events happened in which book from memory, who wrote which story, and so on. If your special interest was the Raiders, you might know exactly who was on the team in 1985, for example, or the scores of every game in a certain year.
Johnny Neurotypical: Oh.
King Tut: See what I'm saying? Autistics tend to get very involved in their interest and soak up information about it like a sponge.