A Casper.js developer designs and implements web navigation scripts using Casper.js, a utility written in JavaScript for the PhantomJS WebKit browser and SlimerJS Gecko. They create automated testing procedures for websites, ensuring functionality and detecting errors or bugs. They also handle event-driven, asynchronous programming, and web scraping tasks. Additionally, they manage webpage capturing and remote DOM manipulation. Their role involves understanding and manipulating various web technologies such as HTML, CSS, and AJAX. They also work closely with other team members to integrate and improve front-end design and usability, providing an efficient, user-friendly interface.