I greatly admire this kind of skill. I'm fine with chord charts and numbers charts - I sometimes write them for bands I work with - but put some actual notation in front of me? Fahgettaboudit. I'd need lots of time to work that up and memorize it.
This isn’t something that came easily for me either. I have been playing out for over 50 years now. Yikes! Like many of us, I learned to play by ear. After playing in rock bands, I began to play with some students and faculty from the University of Iowa music department, playing in a jazzy/pop band. I was a total noob, but I started to learn chord theory and how to play from chord charts.
A few years later, I was playing in a wedding/banquet band. The keyboard player, who was also a bass player, asked me during the break if I could sub for him the following weekend. Thinking that it was another wedding, I said sure. Turned out he wanted me to sub for him on a high school production of Shenandoah. I got the book a day or two beforehand. Fortunately, it is not a terribly complicated bass score. By then, I knew “Good Boys Deserve Favor Always” and “All Cows Eat Grass” but couldn’t actually read, certainly not at tempo. I penciled in the note names above each measure. I knew enough rhythm notation to play basic Broadway Two and Three, quarter half and whole notes. However, it was largely my ear that got me through it. I was sitting next to the pianist, and whatever she played with her left hand, I played. I certainly got some funny looks from the conductor at times, but I guess it wasn’t too bad - she hired me for her next show.
I enjoyed the challenge of playing musicals, and decided if I want to do that well, I'm going to have to learn how to read. I got involved with local community theaters and high school productions, as well as continuing to play in rock/pop, jazz and show bands. My next musical was Godspell. Again, my ear helped me through a lot of it. I’ll have to say, Alas For You with it switching from 7/4, 6/8, 5/8, 4/4, 6/4 almost every measure, required tons of practice on my part.
in my late 20s, after having music as the primary focus in my life professionally, I decided to go back to finish college then grad school, but studying my other interest, biochemistry and molecular biology. During undergrad I minored in music, that gave me a better understanding of Western theory. But, I spent the bulk of my career as a scientist in the pharmaceutical industry doing early phase target identification and validation in cardiovascular and metabolic diseases.
Over the years I just practiced, practiced, practiced, I still try to sight read new material every day. Musical theater is now and has been my primary musical genre. A very active avocation for me. I play for professional regional theater, community, college, school productions and some Off-Off Broadway.