The place to start with questions like this is - naturally enough - Google.
A very quick search
using your subject line as the question gave over thirteen thousand hits:
What is the default behaviour of a controller action not marked with acceptverbs, httpget or httppost? - Google Search[
^]
These included the exact same question posted on Stack Overflow in 2011 as the top result!
In future, please try to do at least
basic research on your own, and not waste your time or ours.