How do we express our negative points to the interviewer?
There are a couple of ways to express something that is contrary to glowingly positive elements to an interviewer. I think that the critical element is how one presents the information in an interview format. Essentially, the person being interviewed is not known, as they are an outsider. Hence, while their insight is valuable to an extent, the lack of familiarity has to be acknowledged. When expressing such thoughts, I would suggest that being able to preface the thoughts with something that shows a sense of constructive criticism evident is going to be important. For example, if the person being interviewed feels that the company recently made a questionable decision, it can be expressed as, "I have always liked the direction of the company. Which is why I am wondering about a recent decision and would like to get your input on it." In a statement like this, there is an acknowledgement of something good, and a willingness to state a "negative" thought along with an opportunity for explanation. It's important for the interviewee to be able to present themselves as not necessarily a final authority, as much as an interested party that can contribute something of value in terms of thoughts and insight to the organization. This would mean moving away from the idea of a "negative point" and rather embrace the concept of constructive thinking. In doing so, the interviewee can show sensitivity and thought while being honest and forthright during an interview.
If the question is referring to how one talks about their own areas of challenge in an interview, then I think that the same principles apply. It is important to be honest in discussing one's own limitations. No one is perfect and I think that savvy interviewers understand the difference between someone who is being insincere in describing themselves and someone who is being authentic in their self assessment. It is important to point out a flaw in one's own approach that is recognized as an area where improvement is being pursued. For example, if one is too intense about what they are doing, the way it could be phrased is to suggest that, "While I know I have a tendency to become very occupied with a project, I am working on being able to take a step back and gain some focus in how I progress with my work." I think that individuals have to be honest about where they see their own need for improvement and be able to state what they are doing or will do in order to become better at it. There can be no right answer here because each individual possesses different faults that cannot be "screened." One cannot say something that they are not because there will come an eventual point where the fraudulent claim will be revealed. Yet, being able to discuss one's own weaknesses and talk about how improvement is going to be evident is a part of professional growth and reflection, elements that are appealing to interviewers and any organization.
I believe you are asking for guidance on interviews for employment. In general it is not necessary to volunteer information on your negative points during a job interviews. There are two exceptions to the rule. One when you suspect that you have a negative point which will makes you really unfit to perform the duties of the job you are being interviewed for. If this is so, you may not be able to hold on to the job for long even if you are selected. In this situation it is better to check out with the interviewer the likely impact of your weakness on your ability to perform the job. The best way to express your weakness in such cases is to express frankly concern about the impact of your limitations on your suitability and let the interviewer take a decision. In any case it is best to describe your weakness in objective and clear terms without emphasizing it too much. Please remember no one in this world is perfect, and people do improve their skills and abilities with experience. Also at times one small negative point may be overshadowed by other positive points. So be honest and let the interviewer decide if you are fit for the job.
Second, situation where it is appropriate to disclose your weakness is when the interviewer asks you to describe your weakness. In this case it is best to own up just one or two weaknesses. Generally one should be honest in selecting the weakness to be disclosed. However, the impact of the weakness can be reduced or even neutralized completely if you also describe the efforts you have made in the past and continue to make to overcome such weakness.
In any case if you are asked a direct question on some aspect that may reveal your weakness. It is best to not to rely on lie or falsehood. In such situation you can reduce the negative impact of your negative points by displaying your resolve and efforts to overcome your negative points.