Filmmaker Nathan Grossman has followed Greta Thunberg’s remarkable journey from the earliest stages of her school strike for climate in 2018 to the United Nations Climate Action Summit last September, often operating as a one-man band shooting quietly in the background with fairly basic camera equipment.

“As the movement grew and she grew, and the scope of the film grew, I thought to myself, ‘Jesus, were we too reluctant to bring in heavier gear?’” says Grossman, who met Thunberg in 2018 through a mutual friend who knew of her plans to strike outside the Swedish Parliament.

The director, whose previous environment-focused work included a series on meat consumption in Sweden, couldn’t have imagined his film “I Am Greta,” which premiered in Venice on Sept. 4, would be among the first major documentaries to hit theaters this fall amid the pandemic. Distributed by Dogwoof, the film is set to play movie theaters in Europe, North America and Australia on Oct. 16, ahead of its release on Hulu in the U.S. on Nov. 13.

Grossman had no expectations when he approached Thunberg about possibly participating in a film. “I said, ‘Let’s give it one or two days, and we’ll go down there and find her and we’ll see what it’s like.’ I asked her if I could put a microphone on and said, ‘Don’t get your hopes up. This is how it works. We don’t know what breadth this is going to have.’”

As it turned out, Thunberg would become a star — certainly of Grossman’s film, but also of a global movement raising awareness of the climate crisis. She would lead rallies of thousands across Europe, thoughtfully explain her Asperger’s syndrome to clumsy journalists, cross the Atlantic to New York on a zero-carbon racing yacht, and eventually become Time magazine’s Person of the Year. And as the film elegantly points out, all of this would be a solitary and often painful journey faced alone.

Variety caught up with Grossman in Venice to discuss his relationship with Thunberg, why Donald Trump doesn’t figure in the documentary, the hypocrisy of government responses to COVID-19 and why “I Am Greta” could help get audiences back into cinemas.

In the early days of filming, how did you navigate forming a relationship with Greta, and gaining her trust?

We have a common interest in this topic…That was a really good start for us to get to know each other, and you spend lots of time together when you travel. On the trains and electric cars, you have so much time around each other when you’re not filming, and we talked a lot about climate change, because that’s her favorite topic.

The film doesn’t focus on the science very much. There aren’t, for example, scenes in which Greta is discussing the crisis in great depth. Was that a conscious decision?

That’s probably one of the things that makes climate change a hard subject to make interesting. We had scenes where she was in Davos and this very famous climate scientist, Johan Rockström, explained lots of things in a panel and it was really interesting. But when we compiled the film and those [scenes] were specifically in it, it felt kind of off when the rest of the movie tried to be more personal. I also felt that you can’t do everything with a film. There’s so much to read about climate change that, sometimes, those facts need to be seen and studied on graphs.

This is one of the first big documentaries that will hit theaters. Is it the right kind of film to do so?

Yeah, I hope so. Our mentality seems to be that we can only focus on one crisis at [a] time, but we need to get much better at not doing that, because every week and month, there are new pollutants getting out into the air and the carbon dioxide level is rising. We need to be able to juggle two crises at the same time, and if this movie can help people get in that mindset during this period, I think it’s really good.

Greta and Donald Trump’s paths did cross at one point, but it’s not in the film. How come?

The film ends in the United Nations and it was somewhere around that time that their paths crossed. I was not there shooting, so we didn’t have any sourced footage. You can make a series about Greta during the time we followed her, because there are so many crazy things that happened during that year. But when we compiled the end of the film, we felt we had to tie together her personal story and not open up a new story about her journey in the U.S. because you would almost have to start a new narrative.

The boat journey to New York makes for some amazing scenes. It seemed harrowing for you all?

These boats are tiny. They look kind of big when you see them in pictures, but because they are race boats they strip them of most of the stuff inside. They don’t even paint them on the inside because they say that adds two kilos of weight. It’s really a boat that is made to sail as quickly as possible.

What was it like filming on the boat during that 15-day period?

I have a background in cinematography, so I’m used to different kinds of situations where you just need to pull together to get those shots. But with the boat journey, it was more like how do you pass that much time? How do you get into a mindset where you feel you don’t need to shoot everything all the time? Because it’s just like Groundhog Day, so [I was asking myself,] ‘How can I save my energy and get into some mindful way of being on this boat because I’m going to be here for a very long time and there’s nothing to look at apart from the horizon.’ That was more of a struggle actually.

How did you guys pass the time?

I slept a lot actually. Greta slept a lot as well. We were good sleepers on that boat. We were maybe cradled by the boat and got sleepy. And we discussed different matters — and life. You’re in this kind of cocoon, in the middle of nowhere, so it becomes a place for reflection.

Do you think Greta might have a greater platform going into the fall, as the world tries to recover from the pandemic and we can reflect on the positive changes we saw in the environment during the lockdown period?

It seems like we can only mentally focus on one crisis at a time, but I think when COVID-19 settles a little bit more, it’s going to be very interesting to see the reactions among young people to how the governments spent millions and millions on the COVID-19 crisis, but they said for a long, long time that, ‘Oh, [the environmental crisis] is very expensive and very complicated.’ It will be interesting to see how young people and others react to that discrepancy in how you prioritize different kinds of crises as problems.