To simulate the behavior of external dependencies, you should utilize mocking utilities provided with the SDK for your chosen programming language. This ensures that tests can run without the need for actual service calls, allowing for true unit testing by isolating the code from external interactions. Creating real client instances during testing would result in calls to the actual services, which contradicts the principles of unit testing. Similarly, configuring API Gateway to manage test dependencies creates actual service interactions. Using sample responses from an object storage service introduces external dependency to tests, which is contrary to the concept of isolation in unit testing.