Witnesses who partied with Anthony during that time said she was cheerful and showed no sign of concern except when her mother would telephone.
They eventually found Casey Anthony not guilty of murdering Caylee, but guilty of lying to police four times during their investigation.
SEE PHOTOS: Casey Anthony: From Arrest to Acquittal
An early vote on the murder charge inside the jury room was 10-2 to find Casey Anthony not guilty, he said.
Eventually, the two dissenters came around, he added, as jurors considered what they felt were holes in the prosecution's evidence and the wording of the murder indictment.
"When the prosecution rested ... I was stunned," he said. "I thought there would be more. I really did. And I was waiting for more. ... A lot of us felt that way, that there was not enough evidence to fill in the gray area that we needed to be filled in."
"We don't know the cause of death," he said in a portion of the interview aired Monday. "Everything was speculation."