"Justin is super private about his relationships, [so] I'm not positive how they exactly met, but they've known each other for awhile, when they were seeing other people," a source told the magazine about the Hollywood actors.
"They recently started hanging out more, going on date-like activities. They're both busy... but they're seeing where it goes."
Long recently alluded to his admiration for Seyfried on Twitter.
"Adjectives don't do justice to Amanda Seyfried's fearless + transformative perf in Lovelace," Long tweeted.