The adaptation downplays the Courtroom Drama aspect of the original, with the story focusing more on Poirot's investigation during and after Elinor's trial. It's to the point where Elinor is found guilty rather than being acquitted before the trial ended.
Poirot gets involved earlier, brought in by Dr. Lord after Elinor confides in him about the anonymous letter. This also means Poirot is able to see Laura Wellman still alive, and after her death he brings up his suspicions of foul play to Dr. Lord. He's proven right about it.
Laura Welman's fling with Sir Lewis that ended up producing Mary is confirmed not just by the letter she had in her possession, but also by the picture of Sir Lewis that Laura kept in her room as well as a photo of Laura carrying an infant Mary.
Since Poirot isn't able to find the vital clues before the trial has its verdict, Elinor ends up getting sentenced to death after being found guilty, leaving Poirot only a few days to clear her name.
Poirot does The Summation and confronts the murderer at Hunterbury, rather than revealing their identity at the trial.
Adaptation-Induced Plot Hole: Zigzagged. While it's initially subverted as the adaptation explains why Elinor didn't drink the poisoned tea by showing that she preferred coffee, the adaptation brings up its own plot hole: why didn't Elinor's defense argue that Nurse Hopkins didn't die from eating and drinking the same things Mary did?
Adaptational Attractiveness: In the original novel, Dr. Lord is said to be "pleasantly ugly", having freckles and remarkably square jaws. In the adaptation, he's played by Paul McGann who has never been described as 'ugly', pleasantly or otherwise.
Adaptational Angst Upgrade: In the original novel of Sad Cypress, Elinor's innocence is proved at court thanks to the clues gathered by Poirot. However, in the adaptation, Elinor is found guilty and sentenced to be hanged in five days after her appeal is denied. Poirot does manage to acquit her, but it's much more angsty that way.
Adaptational Friendship: In the original novel, Poirot and Dr. Lord first meet during the investigation. In the adaptation, they're portrayed as old friends, with Dr. Lord even bringing up the unusual letter to Poirot on behalf of Elinor.
Adaptational Jerkass: Played with.The angelically sweet and innocent Mary Gerrard may or may not be quite as sweet and innocent as she appears since she's more openly interested in Roddy than she was in the novel and more actively steals him away from Eleanor.
Adaptational Karma: In the original story, the killer fled from the trial; in the adaptation, this person is apprehended by Poirot and the police.
Adaptational Villainy: Downplayed with Roddy Winter. Not only does he return Mary's advances more thoroughly, he also expresses some sympathies with the Nazi Party, which annoys Poirot (given that the Germans were why he had to flee his native Belgium in the first place).
Always Murder: Poirot suspects foul play in Laura Welman's death, even if her doctor, Dr. Lord, had reasons to believe otherwise given her recent strokes. Poirot is proven right later on.
Bitch in Sheep's Clothing: Mary Gerrard plays with this trope in the adaptation, in contrast to the original novel; she seems sweet and lovely on the surface and is very well liked by everyone, but she also steals Elinor Carlisle's fiancé away from her and there are some implications that she is cosying up to Elinor's aunt primary to claim part of her inheritance. While Elinor at least clearly seems to think this trope is in play (although, as noted, she's hardly an unbiased witness) you could go either way on whether she's a genuine example of this trope or an otherwise genuinely nice person who just happened to get entangled in a very difficult and painful situation. Nurse Hopkins turns out to be a much more clear-cut example of the trope, however.
Poirot's interest in horticulture (as last seen in "How Does Your Garden Grow?") leads him to reveal that he knew that Zephyrine roses were thornless. This is a change from the original novel where this fact was brought up by an expert in roses who ended up being Adapted Out.
Poirot's dislike for regular tea in lieu of other drinks (he prefers tisanes or herbal teas, for instance) ultimately ends up saving his life when he doesn't drink the poisoned tea the murderer gave him.
Catapult Nightmare: Poirot has one when he dreams of Mary Gerrard, and seeing her face melt away into a skeletal one.
Celibate Hero: Deconstructed. It's hinted that there was some hidden heartbreak in Poirot's past when he tries to console Elinor about her failed engagement.
Poirot:[To Elinor Carlisle, on learning of the ending of her engagement] Madame, I hope you will be permit me to express my sincerest condolences. I too know of the ache of the heart. It is a place most lonely.
Composite Character: Ted Horlick, the gardener who is in love with Mary, corresponds to Ted Bigland, Mary's farmer boyfriend, and Horlick, the gardener who noticed some suspicious activity around the house during the time of murder.
Earn Your Happy Ending: Done a bit more explicitly in the adaptation. After Elinor is released after her acquittal, Dr. Lord gets to drive her home as Poirot looks on.
Faux Affably Evil: Nurse O'Brian in Sad Cypress seems like a cheerful and outgoing person, but pretty much everything she says — particularly on the subject of Elinor Carlisle — is poisonous and waspish, making her a pretty clear candidate for the poison-pen letter writer. Subverted, however, in that while she's clearly not a particularly nice person, she's not evil — and certainly isn't a murderess.
Happy Flashback: Lampshaded. In the opening scene, Elinor is in the courtroom and the last of her thought we get to hear is: "The beginning... the beginning... it seemed happy..."
He Knows Too Much: Attempted toward the end of the adaptation. Nurse Hopkins tries to kill Poirot for letting her secrets out by offering him poisoned tea. Poirot pretends to drink it, and then, in order to make the trap more convincing to her, he pretends to cough a little bit, and then more and more until he gets to the part of choking, making her believe that her plan is working. Just when she thinks he is on the point of death, he manages to gasp weakly that he never liked tea anyways, then pours the poisoned tea into the vase, foiling her plan.
Historical In-Joke: The story's setting in 1937 is reinforced by it taking around the same time George Gershwin died, with his death being shown in the papers.
Jerkass Has a Point: Nurse O'Brian is a poisonous gossip with a seemingly unprovoked malice against Elinor Carlisle. But when Mary Gerrard gripes that Elinor didn't seem entirely pleased to be granting Mary a sum of the inheritance that Elinor received from her dead aunt, O'Brian does point out that any hostility on Elinor's part probably has something to do with the fact that Mary basically seduced Elinor's fiancé away from her, which might also explain why she's not overly thrilled to be giving Mary a huge sum of money on top of it.
Love Makes You Crazy: Peter Lord is already exhibiting this trope in the original novel, but was taken even further in the adaptation: his devotion to Elinor leads him to take actions in her defence that briefly make him seem like he's the actual murderer. It's to the point where Poirot has to call him out for it.
Miscarriage of Justice: In the adaptation, Elinor is actually tried and found guilty, and only acquitted at the very end of the book.
Nightmare Face: In one dream sequence, Poirot tells Mary Gerrard to be careful, but when she gives him a look, her face suddenly melts into that of a skeleton.
Politically Incorrect Villain: The murderer, upon realizing they have been tricked into confessing their crimes, shouts "You filthy foreigner!" at Poirot.
Race Against Time: Since Elinor Carlisle is found guilty and sentence to execution in the adaptation, Poirot only has days to prove her innocence. While he still manages to find many of the vital clues needed to clear her name, he only finds out the most important ones after she's sentenced.
Recycled Premise: Downplayed, but the adaptation sequentially takes place after the adaptation of Five Little Pigs, which also used the "innocent woman tried and found guilty" premise. Fortunately for Elinor Carlisle, however, she's acquitted before she's executed, unlike Caroline Crale.
Undying Loyalty: the one person who possesses complete, unquestioning and utterly immovable loyalty and faith towards Elinor Carlisle is Dr Lord. He refuses to consider even for a moment that she might actually be the murderer even when she's been convicted on overwhelming evidence, and is so devoted to her welfare that he ends up taking actions which accidentally make him seem like the murderer.
You Just Told Me: Poirot corners the murderer by letting slip a fact he could not have possibly known beforehand:
Poirot: Tell me, you have lived in New Zealand? Suspect:[astonished] However did you discover that? Mind you, it is known. Poirot: Is it? I have tried numerous ways to discover it without success. [The suspect looks pissed off and cornered.]
