How was this production of Sondheim's Company casted?

I have always said I didn’t like Sondheim. However, I watched this whole thing last night:

I played in the pit for this musical (Company) and just was curious about how some of the music was written as opposed to how I remembered it, but it sucked me in. I was just gonna skip around, but like I said, got sucked in. Great singing, intimate, convincing acting. Knowing but now over-knowing performances. If you know what I mean.

In this production, the cast plays instruments AND sings the roles. So my question: How is this possible? Was it super hard and costly to scour the earth to assemble this cast?

I keep imagining them in a casting conundrum finding someone who’s purr-fect, but only plays the theremin.
Thanks in advance!

I think a lot of musical people play instruments, and the pool is large so you find people you need.

Raul Esparza, as I recall, did NOT know how to play piano, and had to learn (Sondhiem!) for his performance.

The performance is on DVD, and the commentary/special features are very interesting.