updated 04/03/2012 AT 7:00 PM ET
•originally published 04/05/2012 AT 2:30 PM ET
A decade after Chris Klein found success in films like Election and American Pie, his family and friends knew the actor had a problem with alcohol. But they weren’t the only ones: His dog, German shepherd Chief, realized it too.
“He would get agitated that the alcohol was coming out, and I will be forever grateful to him for that,” Klein tells PEOPLE.
“He began to recognize the behavior shift in me when [I drank] alcohol and his behavior would change. Anybody that has a meaningful relationship with a dog will understand that the disappointment that I saw in his eyes, [it was] insurmountable.”