How are secret service agents expected to respond if the POTUS attacks someone?

One of the US secret service's primary duties is to provide protection to the POTUS, but what if the POTUS were to become homicidal and take an unprovoked violent action against an apparently non-violent target?

What actions would the presidential security detail be expected to take should the President, as an example, start a punching a journalist that recently asked, from the presidents perspective, an offensive and inappropriate question?