I've ben skimming youtube and other sites and found the actors for Yu Seong-ryong and Hideyoshi in past works, in incredibly different roles. These technically aren't related to the show itself, but it mostly has connections with the actors.
Kim Sangjoong who played Yu Seong RYong plays King Gwangjong of Goryeo. THis clip is a preview, but a very powerful presentation of an incredibly aggressive king. A farcry from the dignified scholar we saw him as (though I think its safe to assume his character also started out as righteous and noble, only to take a turn for the worse-Wikipedia says the person was a reformer who purged those against him-sounds familiar); I actually didn't recognize him at first (though maybe its because of his age)
Kim Gyu-cheol who played Hideyoshi, actually played Gwanghae-gun in the Lee Young-ae drama West Palace. The bit is during the court's refuge, with him burdened by the role of the Crown Prince. This guy's been involved in at least three dramas related to the Imjin War. Looks like he can play roles besides weasels.
Great clips, sageuk, thanks for finding and sharing them. That first clip, in case anybody is wondering, is from the 2002-2003 KBS series "Dawn of the Empire" about an early period in the Goryeo dynasty. I had never seen anything from that series before...wow what a bloodbath! You're right, it's not exactly a similar role for that actor, is it.