I'm only familiar with a couple First World War Poets, Siegfried Sassoon and Robert Graves, having read their autobiographical accounts of war service. I would not expect either of them to speak in the fey and affected manner that Bertie Carvell lends to his character Rupert Cadell, a fictional poet and survivor of the war. That said Carvell's portrayal manages to make his character more believable and perhaps less pompous than a straight playing of his lines might have done. I just couldn't think of him as a former soldier but there's no reason why they shouldn't come across as squeaking fusspots. As I said, though, Carvell makes the character believable in almost all areas and he is rather good.
Apparently Patrick Hamilton thought that Hitchcock made a mess of the film adaptation and although my memory of the movie isn't too fresh I think Hamilton had a point. I'm not sure that the all Cadell's speeches in the play were included in the film and I'm not sure how Jimmy Stewart would have brought them meaning if they had.
The play is not without problems, for me, especially the handling of the denouement. This isn't to do with the debate between Cadell and the murderers about whether they are wrong or Cadell's decision about what to do, that was all handled well (acting and writing). It is more the detective story side of things that I thought were fairly ropey (wrote that without realising the pun). There's some really great psychological drama going on (which is something I love in Hamilton's work) but I thought that the setting up of clues was heavy handed, as was the forcing of the confession and just how did Cadell persuade a policeman to hang around outside the house in the few minutes he was out of the room.