The two most common weaknesses that I hear in mockups/VO music are:
a- Inexpressive and Dynamically FLAT Performances/programming
b- Not using the rights patch/articulations for the job
I created this course series (Bringing Virtual Orchestra Music to Life) to tackle these issues with detailed instruction for the intermediate-to-advanced composer.
Volume One (Expression, Dynamics, and Performance) tackles the first problem. In creating this course I went to both my roots as a classical piano student (was gonna say classical pianist but that is a bit of a stretch if I'm honest) and my 18+ years of experience working with sampled instruments to craft both a bootcamp in musicality AND a practical, technical guide on applying these timeless principles of expression to modern computer tools.
Volume Two (Sample Library and Articulation Wizardry) tackles the second by walking you through the common musical jobs/functions that your instrumental parts need to perform, and which sample library or patch types are best for those tasks. I also walk you through the primary articulation types in orchestral music, their traditional purposes, and how they are uniquely expressed, packaged, and controlled in virtual instruments.
But together, I believe these two products present a really robust and detailed bootcamp on increasing the lifelikeness of VO music, so a bundle of the two was asking to be made.