Officers were sent to the AMC Theater after receiving a report about a bag filled with illegal substances, said Fullerton police Lt. Tom Basham.
While at the theater, where "The Signal" was showing, officers were confronted with a crowd of people running out, including two bloodied men.
Basham said both men were randomly stabbed by an unknown man, triggering the audience's panicked evacuation.
"He started stabbing at the theater seat and then he stabbed the victim," Basham said. "As he fled, he stabbed another victim sitting near the exit."
Basham said the victims did not know each other or the suspect. Both men suffered stab wounds to the arm, and one of them had a chest wound, as well.
No arrests had been made but police believe the attacker may have been kicked out of the theater earlier in the night.