O'Dowd, 35, begins to tell O'Porter all about his new role playing the Irish journalist who unmasked Lance Armstrong, with a rapt Kate Walsh, 32, eavesdropping on every word.
For 45 minutes O'Dowd went into great detail about Kate's father's story, interrupted by the occasional autograph hunter. Torn between introducing herself and leaving the actor in peace, she went outside to phone her father. She must absolutely introduce herself, Walsh told her.
"Kate goes back in, stands in front of O'Dowd and says, 'Chris, I'm sorry for intruding but I just wanted to say,' and she paused and O'Dowd said, 'you and your partner would like to have a foursome with us?'
"And Kate said, 'uh, no, just that I am David Walsh's eldest daughter'."
O'Dowd's head fell into his hands and he boomed, "No! No! No!"
The actor made a quick recovery, insisting the pair join them and then buying them drinks, said Walsh.
"He's a very funny guy."
Walsh was impressed by the performances of Ben Foster (Lance Armstrong), Jesse Plemons (Floyd Landis) and O'Dowd. He'd wondered whether O'Dowd could nail playing "a journalist with a bee in his bonnet".
"There's a line in the movie where he [O'Dowd as Walsh] says to his sports editor, 'the man is a cheat!' A number of our kids saw that on the trailer and they said, 'Dad, that sounded so like you'. That's an expression they had heard so many times over the years."
Walsh said the shift in public sentiment had been bizarre.
"You go from the most despised journalist, to the dogged journalist who with great determination kept pursuing the story. A different description of exactly the same thing because perceptions have changed."
The change was ultimately based on Armstrong's confessional interview with Oprah Winfrey, he said, because until he admitted doping, "a number of people were not going to believe it".
His pursuit of the Texan was frustrating and lonely. He felt comfortable talking to only half a dozen of the 500-strong press corps at the Tour de France.
"I was an uncomfortable presence for a lot of journalists. These were people who pretty much knew what Armstrong was doing, who felt they couldn't go there; didn't have the evidence, had to go with the flow.
"I remember thinking at different times that there are always good people out there who are looking to tell the truth. They might be a minority but they are there."
He met up with one of them in Toronto recently at the world premiere of The Program - Betsy Andreu, wife of one of Armstrong's teammates.
"We had a sort of 'wow' moment. We always knew there would be a movie but we never thought it would be this one. We thought it would be one glorifying the heroic Lance."
