There are scenes where the evil doctor is prowling around an apartment complex. This location is actually the historic Los Altos Apartments of Los Angeles, and later becomes the exterior model for Angel's Hyperion Hotel where Angel takes up residence in seasons 2 through 4.
Andy Umberger, who plays the doctor in this episode, also guest starred as D'Hoffryn on Buffy the Vampire Slayer (1997). He also guest starred in the Joss Whedon series Firefly (2002).
Angel now drinks coffee, presumably having taken it up after the events of The Prom (1999) where he tells Joyce he doesn't.
This is not the first time that Cordelia finds herself locked in a basement while her companion duct tapes around the door to prevent pieces of the bad guy from crawling underneath. In season two of Buffy (What's My Line?: Part 1 (1997)), Cordy and Xander barricade themselves in the Summers' basement to escape an assassin demon able to take human form, but who is actually composed entirely of thousands upon thousands of mealworms. The two later kill the assassin by trapping his mealworm form in glue and stomping on him.
This episode is unique in that Cordelia actually tries to 'Scully' Angel as he tries to figure out how Meltzer is constantly watching Melissa, suggesting a non-supernatural explanation, that he may have an accomplice or be using hidden cameras. Angel is actually open to the idea although he points out that it is unlikely as the Powers-That-Be sent Doyle a vision for them to help her.