There are a number of ways of attacking a witness' credibility. Pointing out inconsistencies is only one. And, just because someone testifies to something that is at odds with "reality," that does not mean he or she is "lying." In a trial, the judge will generally instruct the jury as to factors to consider in evaluating credibility and also as to why witnesses may offer conflicting testimony. In California, the jury is generally instructed with, among other things, the following:
"In deciding whether to believe a witness’s testimony, you may consider, among other factors, the following:
(a) How well did the witness see, hear, or otherwise sense what he or she described in court?
(b) How well did the witness remember and describe what happened?
(c) How did the witness look, act, and speak while testifying?
(d) Did the witness have any reason to say something that was not true? Did the witness show any bias or prejudice? Did the witness have a personal relationship with any of the parties involved in the case? Does the witness have a personal stake in how this case is decided?
(e) What was the witness’s attitude toward this case or about giving testimony?
Sometimes a witness may say something that is not consistent with something else he or she said. Sometimes different witnesses will give different versions of what happened. People often forget things or make mistakes in what they remember. Also, two people may see the same event but remember it differently. You may consider these differences, but do not decide that testimony is untrue just because it differs from other testimony.
However, if you decide that a witness has deliberately testified untruthfully about something important, you may choose not to believe anything that witness said. On the other hand, if you think the witness testified untruthfully about some things but told the truth about others, you may accept the part you think is true and ignore the rest."
But, note, as I've said before, if Dr. Stutz testifies that he saw Watson holding Tina in a bear hug, but cannot say he saw Watson touching her tank valve, that might raise a doubt as to whether he turned her air off. (If he turned the air off to kill her and she was found with her air on, that would mean Watson would have had to turn it back on. However, if there is no testimony that he was seen turning it on, that might lead to the conclusion he had not turned it off.) So, the question is whether Dr. Stutz would have been in a position to see Watson turning Tina's air on. Moreover, if Dr. Stutz can put the two together for a period of time that would have precluded Watson from having turned Tina's air off long enough to have killed her, that might also tend to cast doubt on a murder theory.
It could be very subtle. I am reminded of a case a close friend and mentor once tried. The key issue was whether the plaintiff actually owned "stuff" that she claimed was stolen from her in a burglary. At trial she called a friend who was going to testify that she had seen the things in the plaintiff's home prior to the burglary. Before this friend was allowed to testify, my friend was allowed to ask a few foundational questions: (1) Were you and the plaintiff friends; (2) Had you visited the plaintiff's house from time to time; (3) Had the plaintiff visited your house from time to time; (4) Had you called the plaintiff on the telephone and spoken to her; and (5) Had the plaintiff called you on the telephone and spoken to you The witness answered "yes" to each of these. These questions and answers led the court to preclude the witness from testifying! The reason was that the plaintiff had failed to disclose her address or telephone number earlier in the case and thereby precluded my friend from contacting and interviewing the witness before trial and the questions and answers showed that the plaintiff knew where the witness lived and knew her telephone number.
Thus, a good defense attorney could set the scenario up so that a jury would think that in order to have seen what he says he saw, Dr. Stutz would have had to see whether Watson had done something with Tina's valve and that if Dr. Stutz had not seen that, then it didn't happen.
Another point: I would not be comfortable saying that Watson lied when he said he was not taught how to rescue someone. While it is unlikely that such a point was omitted from his Rescue class, it is possible. And, even if the instructor were to say he taught Watson how to rescue someone, perhaps the instructor is not being accurate. Moreover, even if Watson was taught how to rescue someone and forgot it, and based on that forgetfulness, said he had not been taught, that does not equate with a lie. A lie entails being willful.
Yet another point: There have been comments about what the lawyers have said. Do you know how to tell when a lawyer is lying? His lips move. Don't believe what the lawyers say. It is not evidence. It cannot be used for or against a client. Often what a lawyer says is posturing. The lawyer may say something in order to make an opponent think the lawyer has an inside track. Or, the lawyer may say it just so an opponent will spend time and resources being able to address it. Remember, the defense does not need to share any information with the prosecution.
Thus, Watson's lawyer may publicly proclaim that Dr. Stutz saw the DM in a bear hug and saw the DM drop her, just so that the prosecution has to be ready to refute it.