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If you devoured Ozark’s third season right after it premiered in late March, try not to hold Tom Pelphrey’s Johnny-come-lately status against him. “I finally got to see the whole season a week ago,” the actor says, laughing. “I live in the Catskills, and my Wi-Fi is slow, so I had to get very creative about how I could even watch it.”

Pelphrey makes a huge impact on the Netflix crime drama as Ben Davis, brother of budding money-laundering magnate Wendy Davis (Laura Linney). Stemming from his unmedicated bipolar disorder, Ben’s actions make life difficult for Wendy, her husband, Marty (writer-director-producer-actor Jason Bateman), and their drug-cartel employers. Beginning with an introduction in which, as a substitute teacher, he steals a bully’s phone and feeds it into a wood chipper and ending with a climactic episode in which he and Linney go one-on-one for scene after devastating scene, Pelphrey’s work is both white-hot and achingly empathetic, adding new shades to the show’s morally hazy palette. Bad Wi-Fi or no, Pelphrey spoke with eloquence and passion about the ins and outs of his demanding, deeply moving turn.

Your role on Ozark is what I call the Sopranos underboss spot, where you join an existing show and all of a sudden you’re like the main character for the season.

I never felt anything but very comfortable, which is a credit to Jason and Laura and [showrunner] Chris Mundy. It felt like we were all moving in the same direction rather than feeling like I was there to do something specific on my own. That’s a great feeling and also a rare feeling. It’s not every job. When you go on a show that already exists, it’s like being invited into someone else’s home, and you don’t always have the kindest host. But on Ozark? For sure.

Specifically for me, it’s a big credit to Laura Linney. She was so approachable and grounded, so present and prepared and really generous with her time and with her energy. It’s an invisible thing, you can’t really quantify it, but it makes life and work so much easier. It makes it possible to be your best self when that’s the kind of reception you get. That comes from Laura.

You have that remarkable episode where, for the most part, it’s just you and her on your own. You had it all on your shoulders.

A stretch like that is the reason you want to be an actor in the first place. You have these incredible dynamic scenes and you get to play gorgeous writing with one of the best actors there is. That’s the dream. That’s what you’re chasing. That’s what makes all the hard years, the lean years, worth it. That’s what makes all the rejections worth it. You get a chance to do that with someone like Laura and everything is justified.

That episode is where Ben’s bipolar disorder really hit home for me. Bipolar runs in my family, and I’m familiar with that painful dynamic of talking to a loved one, knowing they want to stop acting destructively, hearing them say so and then seeing that they’re totally unable to do it. It hurt to watch, which I think is a testament to the writing as well as your performance.

All of those scenes in that dynamic, the ones you’re saying that landed for you and that you identified with because you’ve seen it in real life, that is 100% a credit to the writing. It was on the page, and then I played it, sure, but it’s the writing.

When you research something like bipolar — I don’t have any experience with it, but I tried to do as much research as possible — you come to understand those thoughts that can perseverate. It’s this endless hamster wheel of a problematic or negative thought and this compulsion to fix things. Understanding that that person can’t do anything other than what they’re doing in that moment helps us have some level of compassion. That’s not a person trying to be irresponsible, that’s not a person trying to make everyone’s life difficult — that is part of what that individual is struggling with. In this context, it’s heartbreaking.

That episode begins with this scene of Ben alone in a taxi, talking to the driver a mile a minute, walking up to the edge of lucidity about his predicament but unable to do anything about it. I’ve read that you stuck to the script in that sequence, but your performance, it felt improvised in the best way.

That was written by Miki Johnson, and I’m sure we’ll all be hearing her name for years to come. Having finally seen what they used, I noticed that there were a few times where I was repeating lines; that must’ve been a certain take where I was just looking for a purchase.

But it was, in my opinion, some of the best writing that I had ever read. Even though on one level, objectively, you’re like, This is kind of rambling and doesn’t fully make sense, I thought that the writer did an amazing job of giving it this flawless emotional logic. Once you can find that, then it’s just a matter of, I want to show up word-perfect because I cannot make this writing better. There’s not a version of me improvising that scene that makes it better. The only thing that could happen with me improvising that scene is making it worse. She’s the writer for a reason, and she’s where she is for a reason, and so you just show up as prepared as possible.

So I spent weeks just going over and over and over the lines because that’s my job. When you know the lines that well, you do really give yourself the freedom to relax and play and let the words work through you, and you go for this ride where you’re not exactly sure what’s going to happen. I just can’t overstate this: It’s the kind of freedom and opportunity that is only possible when the writing is that good, and I really think it was that good.

None of it would work — the whole season really wouldn’t work — if we hadn’t seen an endearing, tender, sweet-natured side to Ben before things went south.

That first scene where we see Ben in the classroom was just such a smart scene. You have this guy who’s kind of fun, who is somewhat intelligent, somewhat has it together but is very protective of what he sees as innocent. The girl who’s getting bullied, he really has a problem with that, and he really follows through on protecting her.

In that scene, we learn this guy really has a sense of right and wrong. This guy has a big heart and compassion for what he sees as victims or people who aren’t being treated fairly. This is a guy who wants to enjoy people, who wants to laugh, who wants to be loving with the people around him. It was blast to play, because at a certain point, it felt like there was no color you didn’t get to touch.