The key to creating a truthful character is allowing that individual to grow from within. Working from the core, wear your character as a thin veil.
Think of an always-changing chameleon, they never mutate their body; instead, they work carefully to manipulate it using the subtlety of their surroundings. In shaping your character, work from the core and allow the environment – your circumstances – to temper your character. Give your character the freedom to exist from within. Shroud the character over yourself as a veil. Don’t attempt to project anything loud or untruthful. Instead, allow yourself to shine through your character, playing them honestly and truthfully. Like a good chameleon, your character will evolve organically so that it reflects both yourself and your environment.
You are the vessel. Shine through the veil.