I found this information in a Lucasfilm licensed book, the Star Wars Movie Trilogy Sourcebook. This is a cool book and has write-ups about the characters. Here is what I found.
Jan Dodonna-- "Dodonna was one of the finest Star Destroyer captains during the days of the Old Republic.
Emperor--"...his old enemy, Mon Mothma". "The Emperor's plans may have reached further back in time than anyone could possibly imagine, for his ability to foresee the future was astounding. Perhaps the Emperor did not destroy [Kenobi] with the rest of the jedi because he foresaw the old man taking young Luke under his wing someday in the distant future. Obi-Wan had failed once and created Darth Vader, the Emperor's greatest servant." Perhaps he also saw Luke's envolvement in the destruction of the death star and knew that this action could create overconfidence in his new found power and that would cause him to make a mistake. <---edited
Owen and Beru---"Young Luke grew up believing that Owen and Beru were his aunt and uncle, but Owen was actually Obi-Wan Kenobi's brother".
Obi-Wan--" Kenobi knew that he could do more to bring freedom to the galaxy by guarding [Luke] than by leading a desperate battle, doomed to failure".
Tarkin--"...who formulated the infamous Tarkin Doctrine of rule through fear..."
Mon Mothma--" This extraordinary woman was one of the original architects of the rebellion, and, to this day, remains its leader and guide. As a member of the imperial senate, Mothma fought to retain whatever basic freedoms the beings of the galaxy had left, as the corrupt Palpatine stripped them away, one by one. The senate was torn, divided, and corrupt, easy prey to palpatine's manipulations, and unwilling to listen to "prophets of doom" like Mothma. And so it was that this visionary leader was forced to take her fight underground.
Crix Madine--"...was an imperial officer in charge of an elite army commando unit."
I am not sure how much of this is supposed to be "cannon" but it all sounds good to me. I thought I'd post to give some insight on Episode III.