Okay. So, actually, there really isn't that much to fear. The worst thing that could happen is they'll invite you to tea, where Hercule will try to impress you with stories of his deeds in the Great War, while Cecil and Pemberton will very rudely interrupt and claim that not only is Hercule too young to have fought in the Great War, but he's also so near-sighted that he would have undoubtedly been stuck scouring pots and pans or mucking out latrines. Wallace will ignore everyone and spike his tea with brandy from a flask made out of an elf skull and regret once again that he didn't become an aerospace engineer like his Great Aunt Clotilda wanted. Don't stay too late, or he'll start warbling off-key show tunes in Runic Dwarvish.
If you decide to go, do ignore the somewhat decrepit state of the manor house. They've fallen on hard times of late, after the rutabaga market crashed in '93. Also, under no circumstances should you agree to play charades. That always ends up in a huge fight, usually with the drapery on fire. Suggest musical chairs instead. But don't let Wallace sing.