A blog (weblog) lets people promptly publish content (such as comments) relating to other blogs through hyperlinks. This type of web content can be considered as a conversation rather than a collection of archived documents. To capture 'hot' conversation topics from blogs and deliver them to users in a timely manner, we propose a method of discovering bloggers who take important roles in conversations. We characterize bloggers based on their roles in previous blog threads (a set of blog entries comprises a conversation). We provide a definition of agitators as bloggers' roles who have a great influence on bloggers' discussion. We consider that these bloggers are likely to be useful in identifying hot conversations. In this paper, we discuss models of blogs and blog thread data, methods of extracting blog threads, discovering important bloggers.