This is cheating, but this is what NPH said in an interview about how he came to play himself in the Harold and Kumar Franchise:
Well, I got a call from a friend who was auditioning for this movie and he was so excited that we were going to be working together. And I said, “I have no idea what you're talking about.” And he said, “You're written in this movie. Neil Patrick Harris is a character in this movie. You don't know anything about it?” And I said, “I have no idea what you're talking about.” And so he said, “Oh, you should have your agents track it down.” So I did, and they read it, and then they called my attorney to find out what was going on, and then I winded up meeting with the guys, kind of cautiously, to see just what their plan was. Because when you're talking about an extreme version of yourself, you want to make sure you're not painted in a super shitty light. Even though it's like dark and funny and whatever, tons of drugs and strippers and lines of coke is dangerous territory potentially. But they were super nice and the whole movie was that same level of absurdity.
Imagine my excitement when I turned the corner onto 8th Avenue while I was in Manhattan, and saw my first White Castle restaurant! No, I didn’t go in.
Anyway, the moral to this very short (and a little bit cheating) post, is that if you haven’t seen these movies, and you love NPH like I do, then do yourself a favour!