The 42-year-old actor posted a picture of Harrelson as Kasady, aka Carnage, Monday on Instagram from the film's set.
The photos shows Harrelson wearing a bright Hawaiian shirt and dark jacket. The character's red hair seems to have been cut since Venom, which opened in theaters in October 2018.
"Hello Cletus," Hardy captioned the post, adding a flame emoji.
Kasady was introduced during the Venom mid-credits scene. Eddie Brock (Hardy), aka Venom, visits Kasady in San Quentin Prison, where Kasady promises to bring "carnage" after he escapes the penitentiary.
Venom is based on the Marvel Comics character of the same name. Brock is a journalist who gains superhuman powers after his body is bound to an alien symbiote.
"It's happening," Serkis tweeted in August. "The Symbiote has found a host in me and I'm ready for the ride... Can't wait! #venom #venom2 #marvel."
Venom 2 opens in theaters Oct. 2.
Hardy stars on the BBC One series Taboo. Harrelson most recently starred in the film Midway.