The visualizer is the main person responsible for the beauty in a project. It is their efforts that bring the image to life.
Their main responsibilities include:
Texturing the scene;
Setting up the lighting setup;
Detailing the scene for different angles;
Preparing channels for post-production;
Rendering previews/final images.
Good visualizers, who handle the entire technical cycle of production from modeling to post-production, are called generalists.
Visualizers are usually generalists and are capable of performing the roles of modelers (and often composers). They can model something for you or do post-production, but it’s better to separate these directions and try to close each stage with a separate specialist.