Attend casting auditions. Let your boss know what's happening. If you're commissioning original music, read lyrics before it's written. Instruments and voices won't change words if you aren't happy with them. Get contract clarification on who controls residual rights to original music, before you pay for it.
Many producers have stock music tracks.
You'll save a lot of time and money. Your producer should identify standby talent in case first choices succumb to the lure of a TV commercial or film job. Building an exciting theatrical experience, over two or three hours, is tough enough on the legitimate stage where they don't stop for marketing speeches and product intros. Ahhh, therein lies the magic of great business theater. I haven't known many writers who could pull it off.