Having studied the play before seeing this film, I can say that is a very good adaptation. The film runs very true to the Sherriff's work, though does exercise poetic license in a few parts in order to make it more accessible to the modern audience. The acting is superb; Northam, Petherbridge and Spall particularly stand out, capturing the harrowing experience of trench warfare excellently. The gritty realism is extremely successful in this particular production, capturing the mood of the play perfectly. As if the excellence needs anymore clarification, the 'Blackadder' lot based the final episode of 'Blackadder Goes Forth' on this particular adaptation (worth seeing as it is funny but devastating at the same time). I won't ruin the end for you, but it's definitely worth watching for.
