"There were sparks right away," the source continued. "They just knew."
Although the pair have not yet wed, Barrymore is already settling into the role of Kopelman's wife.
"She's still Drew, but she has calmed down," the source explained. "It's almost like they're already married.
"It's that When Harry Met Sally thing. Once you find the person you want to spend the rest of your life with, you want the rest of your life to start as soon as possible."
Although the wedding is a top priority, Barrymore is not expected to throw a wild affair.
"It will be very traditional, no elopement on the beach," the insider revealed.
Barrymore and Kopelman are currently working out ceremony details.
- AAP