Far-right extremist Anders Behring Breivik left out no detail from his rampage, explaining how he shot panicked youths on an island summer camp at point-blank range.
Sixty-nine people, mostly teenagers, were killed on Utoya island and others only survived by diving into chilly waters to escape.
Breivik, 33, said he did not anticipate his victims' reactions.
"Some of them are completely paralyzed. They cannot run. They stand totally still. This is something they never show on TV," Breivik said. "It was very strange."
On Thursday, Breivik testified that he sharpened his shooting skills by playing the video game "Call of Duty: Modern Warfare."
In addition to the shooting spree at the governing Labor Party's annual youth camp on Utoya island, Breivik admitted to setting off a bomb July 22 in Oslo, which killed eight people.
Breivik confessed to the attacks, but pleaded not guilty to criminal charges, saying his victims had betrayed Norway by embracing immigration.
The main goal of the trial, now in its fifth day, is to figure out whether Breivik was sane or insane - two official reports have come to opposite conclusions on that point.
The Associated Press contributed to this report.