The Kingsley character's marriage (to Famke Janssen) is falling apart, and he ends up flirting with and then kissing Olsen, who plays another of Peck's regular drug buyers. The hot-and-heavy pairing is broken up by a bartender.
Olsen, after the Sundance Film Festival premiere, said she had been worried about pulling off Kingsley's hairpiece during filming but pronounced the make-out session "fun."
"He was so sweet and so kind," said Olsen, who rose to fame on TV's "Full House" in a shared role with her twin sister, Ashley.
Writer-director Jonathan Levine's film, set to music from hip-hop's golden age, tracks a hot 1994 summer after the main character graduates from high school. He blows on Nintendo cartridges, falls in love with Olivia Thirlby of "Juno" (playing Kingsley's stepdaughter) and listens to Nas instead of his parents.
Rapper Method Man is in two scenes as Peck's nightmare-afflicted rasta drug connection.
"There was no marijuana harmed in the making of the film," Levine cracked.
Kingsley, seen taking hits from a bong and quoting Notorious B.I.G., declared the movie "the journey of an innocent."
"It reminded me of 'Don Quixote,"' said Kingsley, who is well known in India for his Oscar-winning role in the 1982 film "Gandhi."
"The Wackness," which premiered Friday night, was an early buzz movie among festivalgoers and as of Saturday had yet to be picked up by a distributor.