After Mark Grieco scaled the mountains of Colombia just outside Medellin for “Marmato,” the compelling chronicle of a town of 8,000 threatened with being overrun by a Canadian mining company eager to ming the land for gold, he looked towards the sea for his next adventure.

“What’s the opposite of a mountain?” laughs Grieco, when considering what drew him to the Amazon River for this latest project “A River Below.” “Maybe it’s the endless horizon of the largest river on the planet, and what does that look like? What would it be like to film that? I’m trying to build films that are very cinematic visual experiences as well as compelling character studies. I may not have thought of it consciously, but it was nice not to film on a mountain.”

Still, Grieco had many points where he must’ve wondered what he got himself into when he looked into the plight of the pink river dolphin in the Amazon. The filmmaker conducts his own investigation into how this beautiful creature has become preyed upon by local fishermen for its usefulness as catfish bait, but he offers a particularly fascinating perspective on the ecological tragedy by training his lens on two activists who share the goal of saving the dolphins from extinction yet have completely opposite approaches and temperaments. On one end is Dr. Fernando Trujillo, a marine biologist who has devoted his life to preserving the indigenous species of the Amazon, constantly trawling the river to check its health and doing the unglamorous work of forging community partnerships and writing journal articles to systemically raise awareness of the importance of conservation. On the other is Richard Rasmussen, a wildly popular Jack Hanna-esque nature show host who caused a fervor in Brazil when he released video of a dolphin slaughter akin to what Louis Psihoyos witnessed in Japan for “The Cove.”

While the footage is unambiguously horrific, “A River Below” illustrates how murky the water really is in the river as Grieco’s probing of the situation unearths disturbing truths, not just about the predicament of the pink dolphins, but in how their fate is framed in the media, with the gregarious Rasmussen’s influence far exceeding the scholarly Trujillo, though the former’s tactics are far from sound. No matter which of the two you’re drawn to, Grieco makes sure “A River Below” is engaging as a whole, encouraging skepticism of what you’re seeing as he simultaneously creates such an immersive experience of the complicated reality faced by the villagers that you come to have enough context to feel comfortable forming your own conclusions, even if you become uncomfortable with what those conclusions may be. A day after the film’s celebrated premiere at the Tribeca Film Festival, Grieco spoke about what led him on such a treacherous journey, unaware of who to trust himself, as well as what it was like to become a part of the story he was telling and resisting the approach of a traditional social action film.

How did this come about?

Torus [Tammer, the producer] actually is good friends with Fernando, and we started really with his story, looking at what was happening with the gulf and his work there. I’ve been to the Amazon several times and always wanted to make a film there, and this was a great way to start, but we knew from the beginning we didn’t want to make a “Save the dolphin” issue film. So we started with that idea of going with Fernando, and then try to find something much more cinematic, complex and interesting that would be much more of a character driven film rather than just a social issue film.

How did Richard Rasmussen come into the mix?

This “Fantástico” video had actually come out before we started filming, but I first saw the video after starting to film with Fernando, and I realized there’s a whole other side of this story in Brazil because Fernando works in the Columbian Amazon. We went to Brazil to start to see who was affected by this and if people are still killing the dolphins, so we started talking to the fisherman, and they started telling us there was something suspicious about that video. The more we dug around, we finally found this town where the filming took place. It took us a long time to get there, but when we get there they told us it’s this Brazilian TV star [Richard Rasmussen], who came here and filmed this. We had no idea who he was, but he’s incredibly famous there, and once we left that village, and we weren’t sure if they were telling the truth, so we had to find Richard‎. Because I want to give the most fair and objective treatment of every character in the film, but especially someone like Richard, [I thought] how am I going to get that story? So we called him, and our initial conversation was [explaining] we’re interested in what you’ve been doing to try to save the dolphins, and he just told us what he did.

You bring up one of the really interesting elements of the film, which is that you might not have initially trusted the fishermen to tell you the truth about ‎Richard Rasmussen, but they clearly don’t trust you either, as they quickly pull out their cell phones to start filming. What was it like dealing with that initially and eventually becoming a part of your film on camera?

I started off thinking I’m making a film about dolphins, and it ended up being a media study. I was looking at this footage and I’m starting to question what I’m doing as a filmmaker – what we have to do to get a story? What is the truth behind images? Are we comfortable kind of stepping away from it? What’s the power of the camera? Here we find this little fishing village with just a couple dozen people in the middle of the Amazon, which takes days and days to get to, and we show up, there’s no running water, there’s no electricity, there’s no cell phone signal, and they’re all filming us with cell phones. They didn’t know who we were, so they thought now we have proof. Anyone who comes here, we’re going to film them [because] tell us later in the film they had been hurt by a camera, so now they’re going to use it to defend themselves. It’s incredible, and that’s when I step into the film.

Was that a position you were comfortable with?

Definitely not. I don’t want to be the subject, but if I’m making a film questioning what is the truth behind images, then I should be suspect, too. I had to turn the camera on me, or at least let their camera turn on me.

Fernando and Richard become such polar opposites from one another, it’s ideal for such a study. Was it immediately obvious from the start that their stories could parallel one another?

This was really hard to edit. I wanted it to initially look and sound like it was a “Save the dolphin” film, and then it’s going to twist and turn on itself, [but the question was] where does it do that? It’s when we find out the other side of this story with the fishing village, and dealing with a character like Richard Rasmussen, who is incredibly charismatic and very comfortable in front of the camera, [with] an incredible voice and presence. He knows how to use the camera [whereas we had to] balance that person next to the scientist [Fernando] who does not like being on camera. He just wants to do his work, he’s quiet and thoughtful and nuanced. So how do you make that work? I really hope that it works in the sense that they complicate each other in that way.

Was there a real turning point for you, where you might’ve thought you were telling one story and then find out it’s another?

That happens every day. It should happen every day, I think. If you’re going out there really looking for a great story, it’s going to evolve and change in front of your eyes all the time, and the more you stay rigid [in terms] of the story you want to tell, the less you’ll actually find it’s compelling. We started out making a film about dolphins, and it ends up being about ethics, conservation, the role of the camera, the truth behind images, and this shocking footage as an indictment of ourselves that this is the only thing that we respond to to take action. We only believe it when we see it. Then [you have to ask] is it actually real what you’re seeing? The only way to get to that kind of story is to be flexible every day when you’re out there, trying to just see what is the best story to tell.

Though you find yourself in many precarious situations, one of the craziest seems to take place in an airport, just seeing a flock of tourists mob Richard Rasmussen as he’s‎ attempting to explain something about the fish stand inside. What was that day like?

And why do we love that scene? It’s stardom, and we’re seeing behind the scenes of that. We already know this character [Richard] and we feel privileged to be with him, and he’s able to manage that whole situation like a magician. It’s incredible. He’s having a conversation with that person who doesn’t sell the fish anymore, which reflects back on how [his campaign] was all was really effective. In the meantime, he’s doing this other documentary with us, and then he’s able to reflexively stop in the middle of it and take a selfie, it’s incredible [to me] what that looks like on camera. [It was important] to sit there and hold those scenes, hold it on him while he’s posing, because typically, you only see the photo. You don’t know what it looks like to take the photo.

What’s it like getting to the finish line with this and having the premiere at Tribeca?

It’s really nerve-wracking, and it’s been me and my producer against the world really for two years. We had a small budget and an incredibly tough shoot and edit. It’s always tough, but you haven’t slept for two months, and you work 16-hour days every day before the premiere, and last night, it felt like now the whole film team is a family. It’s incredible. Plus, we had a great response, and really what I’m hoping for is that what we had last night continues – that audiences are sitting there questioning themselves, not sure of what is the right answer. We’re not giving you the solution to what is the best way to save these dolphins.‎ I don’t have the answer and this is reflective of how messy life is and I think the film should be a reflection on us.

“A River Below” does not yet have U.S. distribution. It will next play at Sheffield Doc/Fest on June 11th and 13th.