Fans of Buffy the Vampire Slayer, take note: Michelle Trachtenberg, who played Buffy’s mysterious little sister in the final part of that series (and subsequently, for the record, appeared on Gossip Girl), guest stars tonight in a role that won an Emmy for Laura Linney: former First Lady Abigail Adams. Same role, different TV project: Linney’s win was for the HBO miniseries John Adams, while this non-Downton Abby is presented in Sleepy Hollow flashback form. But still…