  The evolution of massive stars is affected by a variety of physical processes
including convection, rotation, mass loss and binary interaction. Because these
processes modify the internal chemical abundance profiles in multiple ways
simultaneously, it can be challenging to determine which properties of the
stellar interior are primarily driving the overall evolution. Building on
previous work, we develop a new modelling approach called SNAPSHOT that allows
us to isolate the key features of the internal abundance profile that drive the
evolution of massive stars. Using our approach, we compute numerical stellar
structure models in thermal equilibrium covering key phases of stellar
evolution. For the main sequence, we demonstrate that models with the same mass
and very similar surface properties can have different internal distributions
of hydrogen and convective core masses. We discuss why massive stars expand
after the main sequence and the fundamental reasons for why they become red,
blue or yellow supergiants. For the post-main sequence, we demonstrate that
small changes in the abundance profile can cause very large effects on the
surface properties. We also discuss the effects that produce blue supergiants
and the cause of blue loops. Our models show that massive stars with lower
metallicity tend to be more compact due to the combined effect of lower CNO
abundances in the burning regions and lower opacity in the envelope.
