I'd say there are two big "stages"
The first is the I-140 approval. This means that the USCIS has determined that you are
eligible to apply for permanent residency, based on your qualifications and your employment, in the employment category. This is usually the biggest "hurdle," so to speak, to overcome.
The next step is to actually apply for permanent residency. This means that you have to be eligible
personally -- you need to not have any conditions that would block your entry to the US, such as being a security risk to the United States, a public health hazard, a public charge, not having a past associated with totalitarian organizations (all those quirky questions one answers in the negative when applying for any kind of a US visa), and also, if you're already in the US, you should have, more or less, been in legal status in the US. (If not, this is does not mean that you're ineligible: just that some things may need to be done, such as CP vs. AOS, or some other things that immigration attorneys know about ...
).
This next stage, after the I-140 approval is either Adjustment of Status (AOS) or Consular Processing (CP). In CP, your file proceeds through the hoops at the NVC, and then goes on to the Consulate. A security and background check is done, I think, at the NVC and the Consulate. You need to appear for medical checkups before the interviews, and then there is the actual interview.
The chances for rejection at the interview, in a "normal" employment based case are low, if all your paperwork is in order, you're reasonably healthy (really, the main public health issues that the US is concerned about are HIV and TB), you're not conspiring to the overthrow the United States
and you're still employed. Appearing for an immigrant visa interview is not the same as applying for most US non immigrant visas. The consular official does not have widespread discretion to reject your case for any reason, or, unlike the NIVs, you appear to be an immigrant. You
are an immigrant, and the rejection has to be for a very specific reason under law.
Of course, recently, depending on your country of origin, the whole issue of the retrogression and the Priority Date may add a long (in some cases interminably long) wait period, making CP much less attractive than AOS. At least with the latter one gets employment authorization and travel documents while one is waiting, or can extend one's H1 if necessary.
When is a GC most likely? I'd say if your I-140 is approved, and your priority date is current, you're well on your way. For those with retrogression issues, of course, your PD has to be current. After that, the interview is generally relatively minor, in most normal cases, as I've mentioned above.
Hope that helps.
