When defining API interfaces in TypeScript, ensure that method signatures accurately reflect the actual behavior and requirements of the implementation. Pay special attention to: 1. Parameter optionality: Consider carefully whether parameters should be optional (`param?:`) or required based on their usage patterns. If a callback is expected to be called in most use cases, it might be better to make it required to guide developers toward correct usage. 2. Promise return types: Methods that return promises must specify the correct type of the resolved value. Never use `Promise` when the promise actually resolves to a useful value. Example of correcting return type: ```typescript // Incorrect getSignedUrlPromise(operation: string, params: any): Promise; // Correct getSignedUrlPromise(operation: string, params: any): Promise; ``` Example of parameter optionality consideration: ```typescript // With optional callback - allows but doesn't enforce callback usage on(event: "validate", listener: (request: Request, doneCallback?: () => void) => void): Request; // With required callback - enforces callback usage for correct implementation on(event: "validate", listener: (request: Request, doneCallback: () => void) => void): Request; ``` Accurate type signatures improve API usability, enable better IDE support, and reduce runtime errors by catching incorrect usage patterns at compile time.