How does ISP prevent "fat interfaces"?

ISP prevents fat interfaces by encouraging role-based interface design — instead of designing an interface from the perspective of a single implementor, you design it from the perspective of each client. Each client only sees the methods it actually needs. For example, a Printable interface with print(), scan(), fax(), and copy() is fat — a simple printer only prints but must implement all methods. ISP splits this into Printable, Scannable, Faxable, and Copyable interfaces. Implementors compose only what they support. The practical benefit: adding a new method to a fat interface breaks all implementors; adding a method to a small interface only affects those that choose to implement it. ISP-driven interfaces tend to be more stable and versioned independently.