I'm not sure if I have officially had what can be considered a "special interest" in the aspie sense.
There isn't anything that I talk endlessly to other people about, but that's because I don't like talking to people about things that I'm interested in unless they are just as enthusiastic as I am.
I have a few subjects that I often crave thinking about. One of them is knitting. I don't always feel like physically knitting, but I like to look at yarn, or imagine working on a project. I keep thinking about making a wool fisherman's sweater, with lots of texture and cables, and being able to wear it in the rain because it's a dense wool that the water just beads up on. Once in a while I go through a phase in which I want to do the actual knitting, and then I have three or four projects at a time.
I think a lot about my Lego movies when I'm making one, and do tend to talk about that a lot. I've only completed one, but it's pretty long, and took ages. I thought of little else while making it. Now I'm making another, but I've been taking an indefinite break from it, because of other obligations. But when I'm working on that, it's hard to think of anything else. It's a remake/spoof of a movie that really exists ("Dark Harbor", 6000 point to any of you who have actually seen it.) I'll be lying in bed, with my husband falling asleep along side of me. Then all of a sudden I'll think of a funny scene that I have in mind, and start laughing like a nut.
My other subject that I just have to think about is my favorite actor, whose name I won't mention, despite an overwhelming desire to do so. LOL! I'll be unloading the dishwasher or something, and I just feel like I need to stop for a moment and think about different roles he's played, and imagine different lines from movies. I'll think about a role that he did back in 1981, and then think of a completely different role that he did in 2006, and I really, really want to be able to say to someone, "Isn't that cool, how he does that thing with his eyes and then he kind of pauses, then draws in a breath there AND there, and even though he's 25 years older in the second one, and playing a completely different character, it's just the same?" and have the other person go, "YES! I do see it! How awesome! How wonderful! Let's get out all of our DVDs and find all the times when he does that!" I guess that's an aspie thing, because it feels kind of frustrating and empty NOT having someone to share that with.
I suppose if I were a bit more AS, I'd talk about my interests to everyone without being aware of the fact that it's driving them nuts. As it is, I am acutely aware of people's reactions, so I know better than to mention anything I'm TOO interested in. When I have to focus on something else, but I WANT to think about a special interest, I feel kind of twitchy inside. I wonder, do NTs ever get that? (Because I'm not diagnosed, and still not 100% sure about myself.)