It wasn't enough for them to write a song that has gotten lodged in legions of brains over the last year or so. (Seriously, at this point "Let It Go" should be paying rent to our heads.)
Now, Robert Lopez and Kristen Anderson-Lopez are about to put up an entire musical inside someone's mind. And it turns out that "Up Here" — the world-premiere work that begins its much-anticipated La Jolla Playhouse run this week — is the one project that has most occupied these married artists' own minds for nearly a decade.
"It's been our favorite show as we've been working on lots of other things all this time," says Robert Lopez. "It's been the thing I really wished people could see through all that time. I'm just really grateful it's finally happening."
Considering the identities of some of those other projects, it's a remarkable statement.
Robert, who made his first big splash (and won his first Tony Award) as co-composer of the 2003 Broadway hit "Avenue Q," went on to collaborate on both book and music for the 2011 Broadway juggernaut "The Book of Mormon," earning two more individual Tonys. (Both those shows also were named best musical.)
Then in 2013 came "Frozen," the animated Disney megahit movie for which the couple wrote 10 songs. The soundtrack went triple-platinum and was the bestselling album worldwide in 2014. "Let It Go," as performed by Idina Menzel, won the pair an Oscar and a Grammy and has sold well over 10 million copies.
On top of that, Kristen has been a key advisor on much of Robert's work and co-created the 2010 off-Broadway musical "In Transit," now in development for a potential Broadway run.
Long before virtually any of that was a spark in their heads, though, the pair were brainstorming ideas for a show about the romantic misadventures of a lonely man named Dan (played by Matt Bittner), as depicted in sometimes fantastical ways via the workings of his own mind.
"It's a very surprising piece, and it keeps evading classification from scene to scene to scene," Robert says. "There's always a new trick, surprise or theatrical conceit coming at you.
"Yes, it's about the inside of your mind, the universe within your mind. But we also live in this even more mysterious universe where we're supposed to communicate with one another.
"It's about one guy's journey from being very much within and introspective and introverted to having faith in being able to communicate his emotions."
To Kristen, a central idea is that "the mind is this prison and this playground. You have this beautiful, kaleidoscopic, ever-changing, fluid world that is your own, but eventually there are those moments that you come up against the walls of it."
And given all that, "Love is a miracle. We hit those walls, and pull in, and then the need to connect keeps us pushing toward those walls again."
As the two chat at the playhouse on a weekday morning before rehearsal, it's clear there's more than a little of their own lives (and minds) in "Up Here."
Robert allows that part of the show's inspiration came from "being a guy like the character in this musical — an introverted, self-thwarting, shy person."
That observation prompts Kristen to recall: "On our first date, he said, 'I'm a shy egomaniac.'" (The memory sends them both into fits of laughter.)
The piece is also informed by Kristen's deep interest in psychology — she once assisted in research on people with schizophrenia and thought she might become a psychiatrist but "got called back by the sirens of theater."
While Robert came up with the show's original concept, it was Kristen who pushed for making it more of a romantic comedy.
The seeds for "Up Here" were planted at the place where the couple's own romance began: the BMI Lehman Engel Musical Theatre Workshop, a New York training program that both attended in 1998.
Robert recalls that on the very first day, "they read us a list of guidelines for selecting your subject matter. And one was you had to [have] a main character who was larger than life, who was not self-thwarting, who had an external goal that was easily definable and nameable, and obstacles that were external, not within.
"I thought, how could you write a show that adheres to these rules and yet completely breaks them and tells the story I want to tell? And I came up with the idea of the consciousness as this big dance ensemble. And the idea of an infinite amount of space inside someone's head was the idea behind a small story told in a big show — the bigger the better."
It wasn't until 2006 — four years after Robert and Kristen were married — that they began writing the piece. The following year, they brought on the Tony-nominated Alex Timbers ("Rocky," "Bloody Bloody Andrew Jackson"), known for his innovative flair, to direct the show.
And some five years ago, the project landed at the playhouse, where Timbers had co-directed the Broadway-bound "Peter and the Starcatcher" (with the late Roger Rees) and directed "Hoover Comes Alive!," both in 2009. (Timbers was last in town at the Old Globe with "The Last Goodbye" in 2013.)
To Timbers, "this show is in many ways an experimental musical. It's really rangy and wild and visual. Elements of every scene take place in an almost fantasy realm. You just don't see that" in most musicals.
The concept includes "these abstract fantasy characters that can lurk and they can watch and they can comment. These things that are always teeming inside our minds, that are good and that are negative. They're always there. And they're always there for Dan too."
Kristen, who likens the show to something like "Woody Allen meets 'Cats,'" notes one other element from her own life that worked its way into the narrative.
While he has never received an official diagnosis, her younger brother shares traits with those on the autism spectrum, and he served as inspiration for a supporting character named Tim (played by Eric Petersen), the brother of Dan's love interest Lindsay (Betsy Wolfe).
"One of the things we're very sensitive to is, how do you have someone who is on the spectrum represented in a show, and really show what it's like to try and communicate and love not be frustrated?" she says.
(Kristen adds that her brother likewise informed the writing of "Let It Go," with its viewpoint on "having a special sibling.")
For these two, taking on responsibility for writing both score and story is "a challenge," Kristen says. "It's hard when there are two of us and we're in charge of the book, in charge of the songs and in charge of these two kids." (The couple brought their two young daughters to La Jolla, where one was promptly afflicted with bird mites, costing the parents plenty of worry plus a day of rehearsal.)
Robert is also venturing into fresh territory with the music, which promises to range among pop, Shostakovich and country.
In an art-meets-life kind of way, the pair's collaboration just might serve as a reflection of the complexities of connection their show depicts.
Robert uses a real-life example to describe what the show explores: "As well as I know Kris, I'm never going to know anything that's going on in her head, and I really don't know her fundamentally, and will never."
To which his wife responds: "And we've been married 13 years!
"And the tools we have to do that with are so primitive," she adds. "Compared with the electrical symphony that's happening in our brains, we have these primitive things like facial expressions and hand gestures and these 'words.'
"It's kind of like playing the telephone game with every other human being on Earth."