Wilson was being interviewed at Leicester Square on Tuesday when a woman outside a pub shouted his name. As he looked up, she raised her shirt and flashed him.
"Wow! I kind of lost my train of thought with that one," the 37-year-old actor said.
In the film, Kate Hudson and Matt Dillon play newlyweds whose domestic life is thrown into disarray when Wilson's character stays too long as their houseguest.
Hudson, who recently separated from her husband, Black Crowes frontman Chris Robinson, did not attend the premiere.
Wilson said playing the character wasn't much of a stretch.
"I went to go stay with Woody Harrelson and his family in Hawaii for four days and it turned into four weeks," he told reporters. "I think by the end of that trip, his wife was like, 'So you need a ride to the airport today?' So sometimes you have to be sensitive to overstaying your welcome."