*Major spoilers ahead – you have been warned*
Released this week, the Marvel adventure is set in the years before Avengers: Endgame and focuses on Natasha Romanoff (Scarlett Johansson) and her family.
Harbour stars as Alexei Shostakov aka the Red Guardian, an undercover Russian agent who served as a father figure to Natasha and Yelena Belova (Florence Pugh) while they were posing as a family in Ohio.
Natasha and Yelena later break Alexei out of jail, but Yelena harbours resentment towards him and in one scene confronts the Red Guardian, telling him that she used to idolise him but is now disgusted by the knowledge that he saw her as a burden.
Alexei is only able to get through to Yelena by singing the Don McLean song “American Pie”, which has featured at the top of the film as a young Yelena asks it to be played when the family flee the United States.
Speaking to Insider, Harbour revealed that the film originally didn’t feature the song at all.
“My character is fumbling trying to be a father and in the end he fails. She tells him to get out. In the script, he says something and leaves,” the Stranger Things star said. “I felt, there’s gotta be something a little more profound.
“I said, ‘Wouldn’t it be interesting if back in America when [Yelena] was little and was terrified having been taken from her family the Red Guardian would put her in the car and drive around and play ‘American Pie’? So from then on she tells daddy to put in the tape.”
The moment was brought forward to the confrontation scene, with Harbour explaining: “[Alexei’s] a failure as a father, what can he do at the end of this scene? [He’s] this narcissist who also has a big heart and so he brings up the song, basically as him saying, ‘I tried.’”
In a recent interview, Johansson revealed details of a costume idea that was “quickly killed” for her character.
Meanwhile, viewers were shocked to find out that the actor playing young Natasha is actually Milla Jovovich’s daughter.
Black Widow is available in cinemas and on Disney Plus now.