One way is through their relationships with other characters. If there's a store owner and an assistant, their relationship could start off rocky but develop into a strong partnership over time. This development can be shown through how they interact during busy hours, when they have disagreements about inventory, and how they support each other in difficult situations.
Characters can be developed by giving them backstories. Let's say there's a regular customer who always buys a certain type of snack. His backstory could be that he used to eat that snack with his late father, and that's why he's so attached to it. This backstory can add depth to the character and make the interactions in the convenience store more meaningful.