In my freshman year of high school I was the backup drummer in our school's jazz/pep/orchestra bands, and quite frankly, I sucked. I couldn't use my limbs independently and I couldn't read music, but I was so into it. I memorized everything the lead drummer was doing, memorized the songs we played, mentally captured everything.
I was too poor to own my own drums, so over the summer I couldn't practice, but it was in my head, and I went over it over and over and over and over again.
First day of band practice my sophomore year, I discovered that the lead drummer had left and it was up to me. I sat at the drumset, and the entire band looked at me questioningly because they knew I wasn't up to the task.
And I rocked.
Probably the best moment of my life, actually.