Ryunosuke Akutagawa, the author of "In a Grove," chose a method of storytelling meant, in my opinion, to show that all people are prone to selective perception and have self-serving truths they tell to themselves and to others. As we look at the characters and their testimony, there are many examples of this.
The woodcutter, one of the less self-serving witnesses, offers details that a woodcutter would notice, more attune to natural elements, such as the kind of trees that were in the grove, a gadfly stuck to the corpse, and grass that has been trampled in a fight. But the gadfly detail might also be offered because it helps to show, as the woodcutter testifies, that the body has been there for quite a while, too long ago for the woodcutter to be suspected.
The priest is quick to point out that he did not notice a great deal about the woman he saw, since, as a priest, he is expected to not pay attention to women's charms. He offers a bit of spiritual and philosophical commentary, in keeping with what we would expect from a good priest doing his job, saying, "Truly life is as evanescent as the morning dew or a flash of lightning" (para. 8), and expresses his sympathy.
The police officer's testimony says more about what a great policeman he is than anything else, as he emphasizes that Tajomaru is "a notorious brigand" (para. 90). He says that Tajomaru is suspected of many crimes, which makes the policeman seem even more of a hero. He does offer some concrete evidence, that Tajomaru has a bow and arrows that were like those of the dead man.
The mother-in-law of Takehiko is completely focused on making sure her daughter's reputation is intact and that she does not become a suspect. She says she is sure that Masago, while "spirited and fun-loving" (para. 12), has never been with anyone but her husband, and she expresses her worry about her, since at the time of her testimony, Masago is still missing.
Each of the members of the triangle, Masago, Takehiko, and Tajomaru, has a motive for murder or, in Takehiko's case, suicide, and each is bent on offering testimony that will save face and honor. The details that are offered are clearly through the filters of each's perceptions of the world. Masago must make clear that she has been violated and that she killed Takehiko to spare him the dishonor of a violated wife, even trying to kill herself over this dishonor. Takehiko is bent upon showing himself as a successful criminal, his stock in trade, and to show that he acted "honorably" within his own world view. Takehiko has been dishonored, and so all that is left for him to do is show that he acted as honorably as he could to defend his wife and then kill himself.
It is important to note that although this story is set in medieval Japan, which explains all the concern about honor, the author's point is universal. Everyone sees the world through his or her own filters, and everyone offers a narrative that is self-serving. I remember a wonderful line from Man of La Mancha, "Facts get in the way of truth," which is, I think, also one of the points of this story. Everyone who testifies offers facts. But each selects the facts. And none of these facts advance the inquiry for the truth of the crime. The author might also be telling us that there is no one truth in any situation, another universal message for us all.