Guards have been watching Williams for unusual behavior and are recording his activities every 15 minutes.
The death chamber where as many as 50 people are allowed to witness executions was cleaned. The chamber was also stocked with fresh supplies of medical tape, syringes and chemicals.
The California Highway Patrol began preparing security measures for the areas outside the prison where hundreds of people are expected to rally.
A federal appeals court refused to block the execution.
Calif. Gov. Arnold Schwarzenegger denied the request for clemency.
Williams, 51, was set to die by injection at one minute past midnight.
The AP contributed to this report.