Choosing a visual style
Before we start designing the characters and environment, we need to set a visual guideline in order to ensure a consistent design throughout the game. When you place the characters together with the environment, they should fit with the background, and they should not have too distinctive art style and quality from each other as if the characters and environment were taken from different games.
Generally, visual style can be split between two categories: realistic and stylized. It's entirely up to you what type of style you want to pick for your game, whether it's ultrarealistic, extremely stylized, or somewhere in between. It depends on the vision you set for your game as well as your team's capability in delivering the art style. Discuss with your teammate what type of visual style they are comfortable with and make sure that you understand your team before you start, which will prevent a lot of troubles from happening during production stage.