“This Is Us” has hinted at Toby’s (Chris Sullivan) private battle with depression since its Season 2 finale last March.

Tuesday night’s episode took viewers on a harrowing journey inside the depths of that battle — and, boy, was it a doozy.

In the episode, Toby — Kate’s (Chrissy Metz) bearded, burly husband — continued his spiral into emotional darkness after going cold turkey on his anti-depression meds, trying to boost Kate’s chances of getting pregnant via fertility treatments.

Toby had been keeping his sinking feelings to himself — not wanting to stress Kate during the long process — while retaining a false facade of cheerfulness. “Thanks for being my rock,” Kate says to a faux-cheerful Toby — as he shuts their apartment door and slinks away into the depths of despair.

“I think a lot of times, maybe it’s not as good storytelling, but depression and anxiety and those things are baffling and can come up out of nowhere,” says Sullivan. “It’s an interesting conversation to have, unrolling [the depression] and the way the pinnacle of all of this [in Tuesday night’s episode] culminates in some ‘joyful’ bit of news as far as Kate’s pregnancy taking.

“Having a kid is a joyful experience — but also, from what I’ve heard, it brings a lot of fear and anxiety.”

Sullivan has, unfortunately, had an up-close-and-personal look at the ravages of untreated depression. “I’ve had friends who’ve dealt with bouts of depression and I just lost a friend [to suicide],” he says.

“It’s a very important conversation we all need to be having right now, a conversation we all avoid because of the ‘shame’ of the topic.

“Right now, with all that’s going on in the world — let’s put it this way, if you look around at everything and you’re not depressed then you’re living a very enlightened life.”

Sullivan is asked about the arc of Toby’s story for the rest of this season —and what the “This Is Us” writers, and creator Dan Fogelman, have in mind for the character.

“I think the theme for Toby, at least for myself, is to have this conversation and talk about mental illness or depression in a way that’s not shameful,” he says.

“As far as Toby goes, I think dealing with these issues in a way that is active and through connecting with people he loves … it’s about becoming the most present and loving and capable parent he can be, as he and Kate continue to face their issues.

“I try not to insert myself into the storytelling part of things,” he says. “They told us that this season is the next stop in bringing more to the Pearsons’ story and giving the viewers a more detailed look at their lives and who they care about — people like Miguel (Jon Huertas) and Toby and getting those stories on the record and hopefully helping the audience mourn Jack’s (Milo Ventimiglia) death … and warm up to Miguel.

“I’m hoping the audience gets a more well-rounded picture of Toby and why he is the way he is.”

Before “This Is Us” came along, Sullivan, 38, voiced Caleb the Camel in those Geico “Hump Day!” ads, which ran in 2013 (“I forgot they gave that camel a name!” he says, laughing). He also co-starred for two seasons as (horse-powered) ambulance driver Tom Cleary on “The Knick” (Cinemax) and appeared as Benny on two episodes of the breakout Netflix hit “Stranger Things.” He also plays Joe on the HBO comedy “Camping,” starring Jennifer Garner and David Tennant.