Twitter Help? About direct messaging...?

[Arlandria]

I tweeted a famous person, Krist Novoselic..if it matters, and he replied. It
was a direct message, like, just for me to see. But..he isn't following me. I found that weird. To my understanding, they have to be following you to direct message with you. How did he do that?
Also, should I reply to him? (Yes. It's the real Krist. Verified and all..)
I asked him a question, and he answered. Here's what he said;
"never!! I am proud of Nirvana. Thank for following me Arlandira!!
- Krist"
I'm debating about if I should reply to that in any way, since it was a direct message. He responded fast, within 2 hours.
I'm not really asking about him in general, I'm just asking you how he contacted me through direct messaging. Does that mean anyone can contact me through direct messages? Should I lock my information or something?
Thanks.

[Luke Marshall]

You dont always have to be following them, its not always the case. Take a look here as to why: http://lx.im/1ce3Z

[Ninetales]

It's because you're following them. As far as I know they don't have to be following you, they can DM you if you're following them (I could be wrong but that could explain why they could DM you).

[Rhiannon Morris]

Hi
Ok, first question first, he was able to send you a direct message because you were following him, he does not have to be following you, however if you want to reply, you will have to @reply him again unless he started following you since you posted this.

Its up to you whether you answer, but unless you have a second question, you might want to leave it, since celebrities on Twitter get loads of replies everyday , either way you cannot guarantee that he will respond again.
Hope this helps
Rhiannon