To access response in subsequent service task make use of parameter mapping of that service task. You use parameter mapping to inject input to the service task and you use result mapping to get the output back from service task.
You do not need to create separate interfaces for every service task, you can reuse it. Define once and make use of it as many times as you want. The same way you can create many methods/operations in the defined interface and make use of them in your service tasks.