Kit Harington has told how he returned to his old school books to help him play a First World War soldier in Testament Of Youth.

The British actor stars opposite Alicia Vikander in the big screen adaptation of Vera Brittain's memoir, documenting her experiences of the First World War and the men in her life who she lost.

The film, directed by James Kent, is screening as part of the London Film Festival in Leicester Square tonight (October 14).

Kit plays her fiance Roland Leighton, an aspiring writer who died on the battle fields in France aged 20.

He revealed: "I did return to the poetry. When I looked at my book shelf I had a surprising amount of war poem anthologies from school - with all my old school note annotation - which was embarrassing. But it did help.

"I mainly referred to the letters. We read Testament O f Youth, but I think what was even more emotionally useful was Letters From A Lost Generation, which is an amazing research tool to have as an actor, actually hearing your characters words. That was the main thing I used.

"But it was lovely to be able to revisit all those incredible poems from the First World War that I hadn't picked up in so long."

The London-born actor - known all over the world as brooding warrior Jon Snow in hit TV series Game Of Thrones - said he felt honoured to accept the role after studying Brittain's book at school.

Kit - who starred in the original West End production of War Horse - said: "When this came up and I read the script, it was such a beautiful adaptation of the book that I'd grown up with, and a character I knew quite well.

"I found it a real no brainer to want to play someone who actually existed and I had so much research material for in letters and the book. It's an honour to be given a real person in history."

The 27-year-old actor also admitted playing Roland as a school boy, before he went to war, meeting and falling in love with Vera, had helped him recapture his own youth.

"I think the way I stepped into it was to try and remember what it was like to be 19, for a start. Because I'm now 27 so I'm playing a guy a lot younger than myself.

"And to remember what that excitement of your first love is... sort of stepping back in time a bit to a cockier, surer version of myself, if that's at all possible."

Testament Of Youth also stars Taron Egerton, Colin Morgan, Dominic West and Emily Watson and has been selected as the BFI London Film Festival's centrepiece gala, supported by The Mayor Of London, Boris Johnson.