Ms Dugdale spent hours behind the lectern playing the First Minister as part of the former Labour leader’s extensive practice sessions for the debates, according to a new book.
The Scottish Labour leader - who now faces Ms Sturgeon on a weekly basis at First Minister’s Questions at Holyrood - was tasked with portraying “statesmanlike Sturgeon” and the “fiery First Minister” during the sessions.
The debate preparations included eight-hour sessions where answers were rehearsed, played back, scrutinised and more research was commissioned to “craft the perfect performance”.
According to the book, one source joked that Ms Dugdale, who was deputy Scottish leader at the time, “had almost developed Stockholm syndrome and was starting to think like the SNP leader”.
“It was challenging for her brain to be in the head of Nicola Sturgeon for hours, as well as providing a critique of Ed’s responses to her,” the source said.
The revelation is included in the book Project Fear by political journalist Joe Pike, detailing the campaign for a No vote in the Scottish independence referendum and the aftermath of the September 2014 ballot.
The book, released on the anniversary of the referendum on Friday, offers an insight into the inner workings of Better Together, which brought Labour together with the Conservatives and the Liberal Democrats to fight for a No vote.
Johann Lamont, who led Scottish Labour during the referendum campaign, and Scottish Conservative leader Ruth Davidson struck up “the most unlikely of friendships”, the book claims, with one observer characterising theirs as an “auntie-niece relationship”.
Elsewhere, former prime minister Gordon Brown and David Cameron developed a “strong, productive working relationship”, the book states.
Mr Brown was given sight of the Prime Minister’s final speech in the campaign, delivered three days before the vote, in order to provide feedback while the two men are said to have spoken on the phone at least twice.
The insights are likely to give further rise to SNP claims that Labour’s near wipe-out in Scotland in May’s general election was punishment for “working hand-in-glove, shoulder-to-shoulder with the Conservatives”.