This is generally a bad idea, the reason why is because you have no control of the life-cycle of an activity that is not the active Activity. You could call a method and find that the activity you are calling has been destroyed to reclaim resources.
A better option would be to create a Utility class that can provide functions to multiple activities, or place things that must remain throughout the life-cycle of your application into the application class, or shared preferences.
If all your trying to do is pass information to a new activity or get information from another one, you may want to package that information into the intent, or again place it in shared preferences.