Generally, you reply to the original post if you're going directly from there. If the post is say a group thread for a party, and Jack and Jill have a subthread where they're talking over by the bar, then if you want your character to be talking to Jack and Jill (and their muns don't mind company), then you would reply to their last comment.
If your character is going to the party, but you are starting a new subthread of them standing over by the karaoke machine, discussing their song choice with someone, then you would reply to the original post, and whoever answered your subthread to chat with your character would then reply to you.