Using the 'Observer Design Pattern' with multiple observers?

In the GoF definition of the observer pattern there is a single subject and multiple observers. This works well because the observers know what kind of data to expect from the subject.

If you had multiple subjects interacting with a fixed set of observers and each subject returned different types of entities/data types, how would you communicate that data to the observers without having to resort to writing very general interfaces that you then have to correctly cast in the observers?