The anticipation was palpable Saturday night as a crowd gathered in front of the Boulder County Courthouse on the Pearl Street Mall just before 8:30.
Suddenly, a zombie jerked her head back and the action began.
Boulder's fifth annual Halloween flash mob helped people get into the seasonal spirit as about 30 costumed dancers got down to Michael Jackson's super hit "Thriller," and hundreds more watched and cheered.
The budding tradition brought some early life to a relatively quiet Pearl Street Mall, despite a Facebook page urging people to dress up and hit the mall as part of the revived Halloween mall crawl tradition.
"What I love about flash mobs is everyone loses their 9 to 5 identities and it's the ultimate freestyle," said Patty "Skull" Shull, who helped organize the event. "I thought it was great. Tonight was the warm-up for Thursday."
Juliet Seskind, Shull's co-conspirator and the zombie the helped initiate the "Thriller" dance, developed the Ayre Dance class at the North Boulder Recreation Center, and said the mob grew out of her desire to do something fun with her class.
"It's a fun way for us to do something together and a bunch of strangers showed up for rehearsal today and that was fun too," she said.
One highly visible mob participant was Boulder's Chris Macor, also known as Mu, the master of the universe. Macor stood out in his long, white wig, and homemade wizard's robe.
"I like to dance," the self-described comrade of Merlin said when asked why he took part in the flash mob.
Macor said donning his costume also presented an outlet for expression.
"It's important for me to come up with this costume because this is a character within myself," Macor said. "I discovered him in my bathtub."
The event, which will be repeated on Halloween night this Thursday, is a mellower tradition than the Halloween mall crawl, which, according to its Facebook page, was also taking place Saturday and again Thursday.
By 6:30 p.m. Saturday, 230 people had confirmed they planned to take part in the mall crawl on the Facebook page but only a few costumed revelers were on the mall early on in the evening, perhaps because the University of Colorado football team was playing on the Boulder campus.
This is the fifth year since the mall crawl was resurrected following a two-decade ban that started in 1991. During it's peak in the 1980s, the event was known to draw tens of thousand of people to Boulder, and cost the city tens of thousands in security, traffic control and cleanup costs.
Emma Mackoy and Caleb Pilon, of Littleton, came to Boulder dawning costumes to go bar hopping with friends.
The couple, dressed as a masquerade ball attendee and "death himself," commented on how quiet the mall seemed.
"It's usually pretty crazy, but it is still early," Pilon said. "I used to come up here when they used to do the naked pumpkin run. That was a whole different scene."
The mall crawl was a family affair for Jane and Dave Roberts and their 7-month old daughter, Ava. The trio dressed up in matching head-to-toe dairy cow outfits.
"We're promoting vegetarianism," Dave Roberts said with grin.
"We love costumes and we love Halloween," Jane Roberts said, noting the family also took part in 2012 when Ava was still in the womb. "Last year there was such and energy and buzz in the air. It's just fun."