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Roger Rees

'Frozen' songwriters unveil 'Up Here,' the musical they couldn't let go

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...