Saw this during the second preview yesterday and wanted to echo everyone else in that I really enjoyed it. I haven't seen either of the movies that this was based on, but I was actually surprised at how well it worked for me as a show overall and how well the two acts fit together. I'll have to watch the movies to see how much of it translated to stage, but in watching the musical I thought I could understand why Sondheim wanted to musicalize this piece (as opposed to Road Show, which I did enjoy but wasn't quite sure what drew him to that story). The production value is great, the set is smart, and the sound design was just about perfect (I was really quite surprised at how robust the orchestra sounded), and the cast of course is doing incredible work, so just based on those factors alone it was worth the trip, and I'm definitely excited to see this show again.
The show proceeds pretty swiftly and I never really thought it dragged, even in the second act. I'm fairly familiar with almost all of Sondheim's work, but it really felt like they were taking a different approach with this one (and maybe that's because he didn't get the chance to fully see it through, but also maybe not). I wouldn't necessarily call it a play with music, since I think the implication with those are that the music is more diegetic, which the songs here are definitely not. It feels more as if they had the script written out (or maybe the broad outlines) and looked for moments that really called out for a song to happen rather than adhering to a more traditional musical theater structure of songs; there's no real opening number (or closing number), no major character I Want/I Am songs except for the more minor characters, no eleven o'clock number, etc. I agree with those who say it would've been fascinating to see more of what a Sondheim/Ives partnership could've produced.
As for the music itself, I definitely hear similarities to Road Show (I had a snippet of a song stuck in my head immediately after the show and then it morphed into Addison's City) and Sunday (more of the style of Gossip rather than the grand sweeping style), though I could've sworn I heard vague echoes of Woods and Company as well. I don't think it'll go down as one of his best scores, but there was something about it where from the first few seconds of the first song it instantly screamed "Sondheim", and I'm really curious to hear it again because I'm sure there are some intricacies I missed.
When reading this thread before seeing the show it did seem strange to me that there wasn't at least a small reprise of the main song at the end of the show, but I actually didn't really feel that it was lacking. The show just moves to such a natural conclusion that I didn't really feel there was space for a song, though maybe a one line reprise could fit. I liked quite a bit of the underscoring in the second act as well, though I agree that the concept seems a bit muddled if they're not going to cut out music completely while the characters are stuck, but I definitely enjoy having it there.
I think the two acts flow together well, arguably better than Sunday, where I think the second act is necessary but the transition into it feels a little jarring. In Here We Are, it's really about the journey of the characters and what they learn along the way (as cliche as that sounds), and act 1 ends in what they think is a respite only to discover there's more to it in act 2. On that note, it's also interesting to consider the two different titles of Square One and Here We Are, which sort of stand opposed to each other, if you think about how "here we are" means presumably that you've made it past "square one" (or maybe you haven't and that says something too).
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The waterworks moment in the second act was both delightful as an Anyone Can Whistle callback, but also a bit surreal given how similar it looked to the videos of the subway station flooding yesterday.