This may be an old topic, I am unsure. I'm also not an expert gamer, so i'll say it the best way I can. I am currently doing the quest "To Kill An Empire" for the dark brotherhoood for the first time on the PC version (I usually have done it on Xbox), and the problem starts as soon as Gianna sees me she cowers then glitches up a bit. Then when we walk to the "Emperors" dining area all NPCs stand up and start glitching where they won't stay seated, thus making it literally impossible to complete the quest considering the "Emperor" won't stay seated long enough to eat the soup. Any help with this bug would be greatly appreciated. By the way, I have the legendary edition and don't use mods yet besides randomly spawning NPCs like Ulfric and Tullius around Skyrim.

you don't need to wait for him to eat/drink the soup, you can Instead just run a sword through his neck as soon as you enter the room he is in..

this also lets you keep the Jarrin Root, as it is an extremely powerful Alchemy Ingredient..