SINGAPORE - With her new film, Scarlett Johansson says goodbye to Black Widow, a Marvel superhero she has played since Iron Man 2 (2010).

The cat-suited assassin was last seen in Avengers: Endgame (2019), in which she sacrifices her life.

Black Widow, the movie set just after the events of Captain America: Civil War (2016), opens on Thursday (July 8).

Speaking to journalists at an online conference, Johansson says she is pleased to be leaving the character of Natasha Romanoff, also known as the Black Widow, at this juncture.

"It's good to go down swinging and end on a high note. I feel fulfilled and satisfied with this movie being the culmination of my journey as Natasha - I did all that I wanted to do with the character and that feels great," says the 36-year-old American actress, twice nominated for an Academy Award.

"If this was the last time I'd be playing Natasha, I'd be happy with that," she says, hinting that the door is open for her character to return in flashback cameos or other supporting parts.

Johansson's Romanoff is a graduate, one of possibly thousands, of a Soviet programme called the Red Room, which trains girls to be killers using a mind control regimen that robs them of free will.

In Iron Man 2, Romanoff has broken free of the mind control and is an operative for S.H.I.E.L.D., a counter-terrorism and intelligence agency.

The new film opens with Romanoff as a lone fugitive running from Secretary of State Thaddeus Ross (William Hurt). He is an enforcer of the Sokovia Accords, created to bring heroes under government control, a ruling she cannot accept.

The refugee is forced to reconnect with figures from her past, including Yelena Belova, played by British actress Florence Pugh.

The two veterans of the Red Room share a past marked by love and also by feelings of betrayal and abandonment.

There are rumours that within the Marvel Cinematic Universe (MCU), the Widow torch will be passed to Pugh.

Johansson said she welcomed Pugh into the MCU fold with a tip: Selling the punch is as important as becoming an actual hard-hitter. It was a lesson Samuel L. Jackson, who plays S.H.I.E.L.D. director Nick Fury, taught her.

"The repetition of fight choreography is exhausting and can burn you out quickly. When I started at Marvel, I was committing tons of effort. Sam Jackson taught me the performance element is even more important than landing the punch," she says.

Pugh's dance background made her "very physically capable", adds Johansson, but preserving one's energy is crucial to making it through the gruelling fight scenes.

Iron Man 2, made before the #MeToo era, positioned Romanoff as a bombshell, an object of lust for Robert Downey Jr's billionaire playboy Tony Stark.

Johansson is known to have asked Marvel to de-emphasise Romanoff's sexuality in recent projects. As the new film's executive producer - her first time wearing that hat on a Marvel project - she spoke about how she used her power to change the dynamic between the two leading female characters.

The project began with the women as adversaries, she says.

"It was an old-fashioned idea of Yelena trying to dethrone Natasha and constantly trying to outdo her. It was disagreeable to me because this is not how women interact. It would be so much more interesting if they had a more emotionally complicated, meaningful relationship, with them trying to lift each other up," she says.

Pugh, 25, thinks the relationship between Belova and Romanoff is marked by tension as much as by affection.

"Yelena forces Natasha to think about her past and realise her actions weren't great. Like little sisters do, she pokes in all the right areas - she forces Natasha to laugh at herself and not be so serious - while also being an assassin."

Australian director Cate Shortland, 52, spoke about how, in the world of the film, both heroes carry the wounds of having been taken from their families as children. In a montage, girls are shown being trafficked and taken to the Red Room.