In a court of law, those testifying are required to swear or to affirm, to tell the truth, subject to the penalty of perjury if they do not tell the truth. As might be expected, when it comes to criminal law, parties of both sides, are going to be prone to not telling the whole truth, because they want their testimony to be structured in a way and manner that this becomes favorable to the outcome that they so desire. Indeed, to believe that courts of law, are actually ever hearing the whole truth, should be seen as the exception, and not the rule in those courts.
When it comes to a criminal trial, those who are members of the jury, have the task of determining which testimony was the whole truth, or a partial truth, or an outright lie, which is not an easy task, but in order to come to fair justice, this determination needs to be made. This signifies that the motivation of those so testifying along with the lawyers regarding a case needs to be looked at, as to their priorities and their objectives. Indeed, it could be said, that everybody involved in a trial, has their own agenda, concerning how and what they are testifying to, of which, it is, for instance, critical for the jury to know, whether anyone so testifying is receiving some sort of quid pro quo, from such testimony, as in a particular governmental witness getting favorable treatment when testifying -- for when this is the case, implicitly or explicitly, such testimony as that, should be seen for what it really is, compromised and not the whole truth. So too, the motivation of a defense attorney has to be considered, for defense attorneys are in the business of getting favorable results for their clients; whereas, the prosecution attorney also wants a favorable result, for their conviction rate is often seen as the signpost of their overall competency, especially in consideration, that the only cases that should go to trial, are those cases in which the prosecution is convinced that a crime has occurred. Additionally, witnesses typically have their own agendas, of which, what they have to testify to, is tempered by their relationship to the defendant. Finally, there is the defendant, who, alone amongst all the other parties, it is the defendant that has to suffer the judgment of the court, which obviously has real consequences, which thereby is indicative that defendants are going to desire to say what they have to say in a way and manner that is favorable to the story that they are telling, and seldom are they going to desire to speak the whole truth, for they want to, make a favorable impression upon the jury, typically, by any means, that they consider to be necessary.
In sum, in a court of law, while those testifying are supposed to provide the whole truth to a given matter, we can take it as a truism, that this is seldom going to happen, so that, any hope of justice, really comes down to the jurors reading between the lines, and understanding the motivations of those so testifying, for those that have the most to lose, will not often hesitate to perjure themselves, whenever they think that they can spin a believable yarn to those that are the jurors.