Joss Whedon has said that in the episode The Body, Kristine Sutherland (whose character had just died) blinked a couple of times when in the body bag. Those blinks were taken out using CGI.

At the Houston Slayercon in 2005, James Marsters told the crowd that vampire blood on Buffy and Angel was actually made of Strawberry Quik.

Glory’s brain-sucking effect was achieved by Clare Kramer pushing her fingers into a dummy head covered in blue material. In post-production, the blue head was digitally removed by computer and replaced by the victim’s head.

The ruins of Sunnydale High seen in Doomed were all computer generated.

When vampires are killed (via wooden stakes, sunlight or beheading), they turn to dust. The Scoobies use the word as a verb to describe killing a vampire (i.e. “We dusted him”). The first person to use the word in this way on the show was Buffy in The Harvest, “So, …

Gareth Davies, a Buffy producer, revealed to the BBC that Glory’s tower at the end of season five (seen in The Gift) nearly became a hazard. He said:
“We found a company which had what looked like a huge oil derrick in its front yard… The Art Department went down and …

Buffy’s cross-like throwing weapon in the episode Anne is called a hunga-munga. It was purchased from a magazine by the props department. The effect of it spinning through the air was created by attaching it to a ball-bearing and a dowel and then spinning it on the spot.

The budget for filming Prophecy Girl didn’t allow for any major CGI, so the Hellmouth’s monster eventually became a massive tentacled thing, made by Special Effects studio Optic Nerve. Each of the tentacles had a person inside it, operating it.

The effect of an invisible Buffy nibbling Spike’s ear in Gone was apparently achieved by gluing a stick to James Marsters’ earlobe, wiggling it about and then removing the stick with computer graphics in post-production.