何时使用存根/模拟，何时在单元测试中使用真实对象?

Is there some rule of thumb or some help how I can decide what dependencies I should stub/mock and for what dependencies I should simply use a normal object? To clarify my question, here is an example code, with different scenarios:

interface DependencyInterface {
    public method dependentMethod() { ... }
}
class Dependency implements DependencyInterface {...}

class ClassUnderTest {
    private $dependency
    public __construct(DependencyInterface $dependency) {
        $this->dependency = dependency;
    }
    public function methodUnderTest() {
        ...
        $result1 = $this->dependency->dependentMethod();
        ...
        $result2 = $this->otherMethod();
        ...
        $result3 = $this->usedInMultiplePublicMethods();
    }

    public function otherMethod() {...}
    private function usedInMultiplePublicMethods() {...}
}

所以我现在的问题是针对测试方法 methodUnderTest 的功能的单元测试，我应该: