Written and directed by John Carney (Once, Sing Street,) the show is based on an iconic New York Times column written by different people each week. It begins streaming Friday.
Hathaway's episode casts her as Lexi, a brilliant, vivacious lawyer who has spent most of her life hiding her condition. Lexi meets handsome, sweet Jeff (Gary Carr) in a grocery store when she is at her lovable best, but the date they arrange for later in the week finds her in the deepest of depressions. Her chronic absences at work also are putting her job in jeopardy.
"I wasn't fully prepared for what it was going to take to represent the highs of the high and the lows of the low and then have to go back to the highs and go back to the lows, sometimes multiple times a day," the 36-year-old actress recently told reporters in New York.
Although shooting the episode didn't take long -- eight days over three weeks -- the role took its toll on Hathaway.
"I really lost my equilibrium by the end and kind of just needed to go to bed and get myself in order," she said. "It made me appreciate that I am an actor. I'm not diagnosed with this condition."
She took the time she needed to decompress after the project ended, assuring herself, "This will be behind me, which is a very different experience than having a condition that you don't have control over."
The star of The Princess Diaries, Brokeback Mountain, The Devil Wears Prada, The Dark Knight Rises and Les Miserables -- who is pregnant with her second child -- said making Modern Love was an eye-opening experience, challenging though it was.
"I don't like actors who complain and go on about how exhausting their parts were," she said. "We are the luckiest people in the world, and if you are a working actor, that's your job and be grateful. There are a lot of people who want your problems."
To convey to viewers in a short length of time the extremes of Lexi's radically shifting moods, the character is seen beautifully dressed, singing, dancing and charming her way through life in some scenes, and then barely able to move, speak or get out of her pajamas in others.
"Every choice has to matter," Hathaway said. "Every visual cue had to tell you something about her immediately because there wasn't time to explain absolutely everything to the audience."
Because a perceived stigma often is attached to mental illness, many people avoid seeking treatment or confiding in their loved ones about what's going on, the actress said.
"I'm not saying this is the episode that is going to change anything," she said. "I don't think it's a magic bullet and I don't think it is the answer, but I am happy to be a part of a show that, in its own imperfect way, invites more people to the table."
Each episode of Modern Love tells a different real-life story about New Yorkers in unexpected relationships. The show's revolving cast also includes Tina Fey, Dev Patel, Andrew Scott, Brandon Kyle Goodman, John Slattery, Catherine Keener, Julia Garner and Cristin Milioti.
"There is a real strong tendency among a small group of people to dismiss shows like this, to dismiss this genre, to not value it the same as dramas and gritty things," Hathaway said.
"Context is everything, because in a year where everything is going good, this just feels like fluff. But, in a year like 2019, where we are all gasping for positivity, this feels like a lifeline."
Known for being candid and self-aware in interviews, the actress credits her upbringing for her feelings of ease with being herself and expressing her opinions.
"I wasn't raised with too many expectations about how I was supposed to behave," she said.
"I really, really was raised in a very free-range environment, especially emotionally, and, so, I don't think of it as speaking out. I just think about it as representing the world and the people that I know and love, which doesn't feel particularly brave or inspired or important until you realize that somebody has felt trapped because they weren't raised in that free-range way."