Scott D. Pierce: Mark Ruffalo is great as twin brothers in dark HBO drama. And there are happier things to watch, too.

(Photo courtesy of HBO) Mark Ruffalo plays twin brothers in the six-part HBO drama "I Know This Much is True."

Mark Ruffalo doesn’t turn in one great performance in the HBO limited series “I Know This Much Is True,” he turns in TWO great performances.

Based on the 1998 book by Wally Lamb, this harrowing six-part series (which begins Sunday at 7 p.m) is about identical twin brothers Thomas and Dominick Birdsey. Thomas is a paranoid schizophrenic and Dominick tries to protect and care for him.

Ruffalo plays both brothers, and he had to eat his way into becoming Thomas. Seriously. Because of the various drugs Thomas is on, he’s heavier than his brother. And, after finishing his scenes as Dominick, the production took a five-week break while Ruffalo went home and ate and ate … and ate, gaining 30 pounds to play Thomas.

“We didn’t want it to be, like, I run and put a wig on, and then run and do the same scene on the same day,” Ruffalo said. “It was important for us to really, really have these guys be two different people.”

Mission accomplished.

“I remember when Mark came back to set as Thomas, the crew was in a state of awe and shock,” said director Derek Cianfrance, “because he was a completely different guy.”

It is startling to see Ruffalo’s transformation. Astonishing, actually. When I first saw a clip of “I Know This Much Is True” several months ago, I thought, perhaps, some kind of CGI was involved. That would’ve been easier on Ruffalo, who said it was “really challenging” to put on 30 pounds in a month.

“I didn’t expect it to be. I thought I was going to be having a fun time doing that,” he said. “And when you’re force-feeding yourself, some of the romance of food sort of leaves.”

Make no mistake. There’s nothing fun about “I Know This Much is True.” It’s about severe mental illness — Thomas cuts off his own hand just minutes into the first episode — and the consequences not just for Thomas but for Dominick and the rest of the family. And Dominick is not just dealing with his brother’s problems, but with some pretty severe issues of his own. Those play out as he makes some unexpected discoveries about his family’s past.

It’s an engaging drama and Ruffalo’s performance … performances are fantastic, but “I Know This Much is True” it isn’t easy to watch. Be prepared for that.

If you’re looking for something lighter or something to inspire you, there are several new offerings this week, too:

“Jimmy O. Yang: Good Deal” (Streaming on Amazon Prime) • OK, I’ll admit it. I know Yang from his roles in HBO’s “Silicon Valley” and the hit film “Crazy Rich Asians,” but I hadn’t seen him do stand-up before.

He’s freakin’ hilarious.

Yang talks about his immigrant parents, being Chinese American in 2020 America, battling stereotypes — and he’s both funny and smart. And in the middle of the pandemic, finding something that’ll make you laugh for an hour is a gift.

“Amy Schumer Learns to Cook” (Monday, 11 p.m., Food Channel) • Sequestered at home during the pandemic, Amy’s husband — chef Chris Fischer — tries to teach her to cook.

All I’ve seen is a clip from the second episode, but I laughed more and harder in those 5-6 minutes than I do at most half-hour comedies. And at most two-hour movie comedies. I cannot wait to see more.

“The Happy Days of Garry Marshall” (Tuesday, 7 p.m., ABC/Ch. 4) • “Legend” is a term that’s tossed around too easily in show business, but Garry Marshall was one. The TV shows he wrote and/or produced included “Make Room for Daddy,” “Gomer Pyle,” “The Lucy Show,” “The Dick Van Dyke Show,” “The Odd Couple,” “Happy Days,” “Laverne & Shirley” and “Mork & Mindy”; the movies he directed included “The Flamingo Kid,” “Beaches,” “Pretty Woman,” “Runaway Bride” and “The Princess Diaries” — and that’s just a fraction of his credits.

I loved him as an actor — as the crazy network president on “Murphy Brown” and the family patriarch in the TV movie “The Twilight of the Golds.” I also loved interviewing him over the years — he was funny, charming and clearly loved what he did.

This star-studded, hourlong tribute is a little late — Marshall died almost four years ago — but it’s much deserved. And it’s fun to watch.

“Becoming” (streaming Wednesday, Netflix) • If you’re a fan of Michelle Obama, you’re going to love this documentary. If you’re a fan of her husband’s successor in the White House, odds are you won’t.

And that’s unfortunate, because this 90-minute program is charming and inspirational. It not only shares its title with the former first lady’s best-selling 2018 autobiography, it is, to a large degree, a filmed version of that book. And it’s combined with footage of Obama’s book tour to promote the print version.

There are no big revelations. She’s not running for anything. Which will disappoint the people who love her and come as a relief to those who don’t.