Cruel thing about life is that we cannot learn how to do things in right way but we only pile up cases which should not be. In other words, we learn and grow by errors. This is fairly sad thing to encounter every occasion, but also reveal why experience is so critical in human wisdom. So, for the grand question of how to do PhD - an everlasting question in self-perpetuating world of academia repeating every year - may be answered not from falsely righteous advices, but from bad examples in lamentable memories. Here, I hope to share few common cases that I have seen in grad school (indeed anonymously - nobody wants to hear sad story with name tag). Wish debris from the past be stepping-stones for new comers.
Case 1 - Graduate "student"
This is a fairly common case. Although the word graduate student ends with "student", the position is kind of fairly mixed version of student and researcher in reality. So, the graduate study is continuous cycles of learning something new as a student and then apply it into his/her own problem as a researcher. But, occasionally, there are people who just go half of the cycle, and repeat it - just study as a student. It is fine to do this for Master's degree, but this is most of time quite a big trouble for doctorate.
Case 2 - Tenured graduate student
This is also common but more tragic than the first case. PhD study is supposed to end around 5 ~ 6 years (although depends on disciplines) - starting from basic training from courseworks, qualifying exams, thesis topic proposal, few rounds of committee meeting on progress, and defense - a virtuous metamorphosis from a 1st year grad student just finish their commencement to an young scholar who can study their favored topic more or less independently. For many reasons, this natural cycle goes wile sometimes, generating an unholy creature so-called tenured graduate student (actually not tenured indeed, but just exist too long in the department as like tenured faculty).
Case 3 - Dunno volume conservation
This is kind of recent as multidisciplinary study has been popular in many disciplines. Even for smart graduate student (except truly exceptional ones - yes there are such rare species), one person's intellectual capacity is more or less limited especially during 5 ~ 6 years of graduate study. Assuming a person's intellectual capacity is volume, then depth of it would lower if it spreads too much - which I find analogy with volume conservation. PhD should have certain level of depth in their thesis topic, which is basic requirement in most of disciplines. However, sometimes people spread themselves too much rendering their depth of knowledge way too shallow. It is important to know his/her own intellectual capacity (his/her own - this part is important as intellectual capacity is a quality varies astonishingly a lot between people), and confine their range of coverage during PhD study.
Case 4 - Amateur professional
This is the worst case I say. PhD is not just a degree but a course of training & a process of maturation into an independent professional of his/her own field. There are rarely people who don't have virtue of acknowledging their own level of maturation - most of time confusing other people's contribution to their own contribution. Academia is kind of river in which knowledge and skills flow from senior to junior researchers over generations to keep their own versions of cherished light of intelligence survive. However, due to unfortunate personality disorders, there are people who simply confuse what they can do and what they actually cannot do. I call such kinds as amateur professional - in a sense that amateur but believe themselves as professional. For this bad case, the basic mechanism of PhD doesn't work out at all as they refuse to be part of it. For Case 1-3, good advisors or help from the department may save the situation to finalize PhD. But for this case, there is no remedy. If you find somebody in this case, the best consulting would be finding a better position in somewhere else (although I wonder whether this would be welcome in other non-academic positions or not).