Any casting directors here? As a singer, how much knowledge of sheet music do I need?

Are you're asking if, during an audition, you will ever be handed sheet music for a song you may not know, and then have to sing it? There is no reason for them to do that. The audition is for them to hear you sing a prepared number. They are also after the right look for the parts they have, as well as the right sound. It doesn't hurt if you can read music. At least enough to follow and learn a part following a recording.

My family was in a community theater production of "Anything Goes". Great fun. Hope you get to finish it.

There are plenty castings for which you only have to prepare songs known in advance.
Though there are a decent amount as well where you're handed your audition score just an hour in advance; there is no shame at such a point in time to tell them that you can't read sheet music, but that you're willing to try singing along as best as you can and guarantee them that you'll always be well prepared before rehearsals.

Yes, being able to read on sight gives you an edge in such cases; but in my experience that part of the audition is really only taken into consideration as a tie-breaker. There is so much more to casting that just the capability to sing on sight.

Hi, barencor –

I've music-directed a few dozen musicals at high school, community, university and semi-pro levels. Here are some thoughts on that:

In musicals, the director's goal is to cast actors who will look and sound great performing the material. Not every talented actor (or even every great-sounding singer) arrives with the ability to learn and sing parts quickly. And if they can, their understanding of music theory shows more plainly as music becomes more harmonically or rhythmically complex.

Some of the world's most recognizable music theatre celebrities (e.g. from Broadway or movie musicals) aren't the world's most intellectual musicians, but they're achieved that status because their talent and presence was worth the recruiting of whatever musical coaching it took to support them. That shows the paramount importance of listening to your heart and your gut when acting, and I'll put that aside.

In community theatre, skill varies, resources are scarcer, and directors have to strike a balance between the available talent and the available time for support from their music director, accompanists, and so on – so the more skill evident in individual cast members, the better, period. That makes being excited about increasing your music literacy a near-certain path to advantage. After casting, many artistic teams will provide a recording as a learning aid, but every artistic team should provide a vocal score, and I'd consider it a minimal requirement to be willing to use that score as an educational opportunity.

If there were no recording, you could still notate your part in MuseScore by directly copying the vocal score, and use its playback to listen to it. The value of that exercise in preparing you for the next musical rivals that of performing in the show, in my opinion. If you take one measure beyond using MuseScore, I'd suggest learning to play your own vocal part on a piano or keyboard; equally valuable practice.

You can even use this ability to notate and create guide vocal parts for other actors; when they start talking with each other and the artistic team about how helpful you are, you could significantly boost your reputation as a valuable person to cast within that community.