I want to check some things about the state of the session, the user agent, etc, and possibly take action and return a special view BEFORE a controller method gets a chance to execute. For example:

Most common:
User requests Home/Index
System checks to make sure x != 0.
x does not equal zero, so the Home/Index controller executes like normal.

But, sometimes:
User requests Home/Index
System checks to make sure x != 0.
x DOES equal zero. The user must be notified and the requested controller action cannot be allowed to execute.

I think this involves the use of ActionFilters. But I have read about them and I don't understand if I can preempt the controller method and return a view before it executes. I am sure I could execute code before the controller method runs, but how do I keep it from running in some instances and return a custom view, or direct to a different controller method?