You can go to User CP and edit Options, then choose instant Email notification for new replies to that thread. To get notified automatically when a particular poster makes a post is not possible (I think). Maybe some other posters know of other notification options.
wil.
<FIELDSET class=fieldset><LEGEND><LABEL for=sel_autosubscribe>Default Thread Subscription Mode</LABEL></LEGEND><TABLE cellSpacing=3 cellPadding=0 width="100%" border=0><TBODY><TR><TD>When you post a new thread, or reply to a topic, you can choose to automatically add that thread to your list of
subscribed threads, with the option to receive email notification of new replies to that thread.</TD></TR><TR><TD><SELECT id=sel_autosubscribe style="VISIBILITY: visible" name=autosubscribe> <OPTION value=-1>Do not subscribe</OPTION> <OPTION value=0>No email notification</OPTION> <OPTION value=1 selected>Instant email notification</OPTION> <OPTION value=2>Daily email notification</OPTION> <OPTION value=3>Weekly email notification</OPTION></SELECT> <LABEL for=sel_autosubscribe>Default Thread Subscription Mode:</LABEL> </TD></TR></TBODY></TABLE></FIELDSET>