Special effects designer William Forsche recently posted a ton of rad behind-the-scenes footage from the production of Ghostbusters II on his YouTube account. Witness puppeteers manipulating Slimer (begins at 4:04) and the Scourge of Carpathia acting stern.
You can also view several more of Forsche's clips from the film, including a video of Bill Murray and Ivan Reitman hanging out and Vigo's make-up and the Ghost Nanny. Here's the video of crew filming Vigo as a talking painting. Notice that his real voice isn't super-imposing.
[Via Bloody Disgusting]