Between his strong new album – (pronounced Subtract) and the week he spent in court successfully proving the originality of “Thinking Out Loud,” Ed Sheeran and his songwriting methods are under the sharpest scrutiny of his career. Other than the man himself, few have more insight into that subject than Snow Patrol guitarist Johnny McDaid, who’s been collaborating with Sheeran since 2013 as a co-writer and producer, including on the recent hits “Bad Habits” and “Shivers.” In an interview conducted for Rolling Stone‘s Sheeran cover story, McDaid, who’s also close friends with Sheeran,...