How to retrieve form values from HTTPPOST, dictionary or?

I have an MVC controller that has this Action Method:

[HttpPost]
public ActionResult SubmitAction()
{
     // Get Post Params Here
 ... return something ...
}

The form is a non-trivial form with a simple textbox.

Question

How I access the parameter values?

I am not posting from a View, the post is coming externally. I'm assuming there is a collection of key/value pairs I have access to.

I tried Request.Params.Get("simpleTextBox"); but it returns error "Sorry, an error occurred while processing your request.".