About 20% of Tremor Video advertisers are now buying ads on a cost-per-engagement basis, up from virtually none a little more than a year ago, says Lauren Wiener, President, Global Sales and Marketing at the digital video ad technology company during an interview with Beet.TV. In mobile, even more marketers are opting for cost-per-engagement pricing so they only pay when a user "leans forward and engages with the ad," Wiener says. "There is a big concern in mobile that the ad could be running and no one is watching it," she tells us. Cost-per-engagement protects against that concern.The spending growth in digital video has been complementary to TV budgets so far, and some advertisers are even using online video as equivalent to a TV make-good, she tells us, underscoring the cross-device reach of video. Wiener does, however, expect money for digital video to come from shrinking shares in other mediums such as radio, print and display.Looking ahead, more marketers will use interactivity in video ads. "We are working more deeply with creative agencies to add Facebook sharing or Twitter feeds, or outtakes from commercials they have shot. They're looking for something to make the TV creative more shareable or tell a deeper story," Wiener explains. That type of interactivity might even include graphic overlays in car ads that let the users see the car in other colors, for instance.