I didn't completley understand your first question! But that laughing thing happens to me too. I used to think like: I'm not going to be able to laugh, someone funny is going to happen and everyone will laugh except for me, and I kind of laughed for like a while but forced, but sometimes my friends kept seriously laughing and I couldn't keep it up. If I don't find the joke funny, I try to think of the situation! For example, I see that everyone else is finding it funny but me, and I think that they are thinking that it is hilarious but it's not that funny at all, and that sort of makes me laugh! Then I for example can think that everyone else is finding it funny except for me, and I'm going to laugh at that, and everyone else is laughing at a different thing then me, and that is also funny. There are a lot of things you can think of hehe.
Other thing is that once I am going to start laughing I get intrusive toughts like: "I'm not going to be able to laugh" or "I did something terrible in the past how can you dare to laugh?" something like that. That kind of bugs me sometimes, but if you ignore it and laugh through it it won't matter, you'll later on thank yourself for that.
But the most important to me is that I don't HAVE to laugh, if I don't find it funny. "Too bad you think I'm boring guys, I'm still in a good mood anyways, no reason to find me boring." something like that. And that dissinterrested thing with people happens sometimes too, but I just don't have anything else to say, doesn't mean that I am not receptive. In real life even if that happens my friends usually can tell that I am not dissinterrested, it's no big deal, they still usually want to talk to me, because I always try to be in a mood to have fun. Even if we are not talking we still enjoy hanging around with eachother, because we're all always feeling well, almost every conversation is fun!
That sort of thing (Laughing etc) just happens naturally if you're in a good mood in my opinion! And ignore the intrusive toughts! I don't have like a "script" to explain this one better, but if you improvise on the spot something you can still be fun in any situation! Well this probably hasn't really helped you, but nice to know someone else also has the same situation as you hehe.