Lincoln's Assassination... and his prior disability
At roughly 10:30 p.m. on the evening of April 14, 1865, actor John Wilkes Booth fired one bullet from his .41 caliber Philadelphia Deringer pistol from point-blank range, bringing the administration and life of 16th President of the United States Abraham Lincoln at 7:22 the following morning. The assassination was just one aspect of what today would be described as a "decapitation strike attack" on the United States Government, with a second element (the attempted murder of Secretary of State William H. Seward) taking place almost simultaneously. Rather than carry out the third element of the plan (the assassination of Vice President Andrew Johnson), conspirator George Atzerodt would get drunk and wander aimlessly through the streets of Washington, D.C.
There being but eight hours between the time of Lincoln's shooting and his death, little thought was given to the prospect of Vice President Johnson acting as President. But a year and a half earlier, a discussion of presidential disability probably would have been in order.
In November 1863, Lincoln would fall ill with what eventually would be diagnosed as a severe case of smallpox. Laid up for the rest of 1863 and into January 1864, under modern conditions Vice President Hannibal Hamlin would have been summoned and at least a discussion of Hamlin assuming some of Lincoln's responsibilities, either on a formal basis as Acting President or, at least, on a ceremonial level.
As it was, the Vice President was serving the Union's war effort as a private in the Maine Coast Guard - an indication of just how important everyone, including its holder, viewed the office of Vice President during that era.