The Kremlin has unleashed a new weapon in its information war with the West: the fake celebrity cameo. “Hi, Vladimir, Elijah here,” the actor Elijah Wood said in a video packaged to seem as if Mr. Wood were addressing Ukraine’s president, Volodymyr Zelensky. The actor, best known for playing Frodo Baggins in “Lord of the Rings,” urged the president to enter treatment for drug and alcohol abuse. “I hope you can get the help you need,” Mr. Wood signed off. The video was recorded on Cameo, the popular, though now struggling, app where users can pay for personalized messages from...