FireFly you must have the same checks or this file will become a way for users to look at hidden threads, for example.
I stripped off what clearly isn't needed (like go to next/last thread, like go to newest post, etc).
But you still have to make the same "work" as you would for showthread:
check in which order to show the posts - newest first or oldest first?
check whether the thread is visible
get permissions
do the posts-per-page routine
draw the navbar
bbcode et. al.
page nav
admin options or not
In short, you treat it almost exactly like you'd treat a thread. Well, it IS a thread. Just only one poster's messages in it