You can get
something like what you're talking about simply by using Threaded (vs Flat) Mode, which is set in your preferences. It won't expand the entire forum, but it will for each topic independently so you can see everyone who posted and when.
(Personally, I hate Threaded mode -- I like to see the entire conversation on one page rather than having to click 100 times to read a conversation. But to each his own.
)
