I answered the same question over on another forum but will add something here because of the different responses/perspectives.
I primarily model scale passenger cars i.e. 21" versions or GGD cars, which are slightly shorter, as are the Lionel Superliners. With a standard set of 6 - 8 cars, you could spend a lot of time and $ in fully populating a complete train. My sets are primarily from the streamliner era when you could expect a fairly full train.
However, you are constrained in numbers partly because of the lack of availability and variety in passenger figures that readily fit in O scale interiors, which are actually sub-scale and closer to S scale than true O. MTH Railking figures and the old K-Line figures (some of which are now issued by Lionel) are the best if you don't want to have to perform radical plastic surgery on the figures.
Partly as a result of that, I have only fully populated certain special feature cars on which I am making a lot of modifications anyway. Most recently, I have worked on the UP dome lounge observation car that is part of the Excursion Train set. Spoiler Alert - my interiors are basically fantasy schemes with only some fidelity to the prototypes, as in a nod in the direction of nostalgia. To compare T. Albers' fine K-Line dome car with what I have done, here is the dome section:
I had to add both ceiling and floor lighting to the dome because the Lionel cars come without any. All the table accessories are "quarter scale" 3D printed items of which there is a vast amount for diorama or dollhouse creators.
The Excursion car domes are prototypical with very large curved glass panels. This makes life a lot easier than older domes because of extra headroom. In contrast to the above, in an earlier 21" Lionel dome car with a conventional framed glass roof I put in fewer people both in the dome and below and again added lighting. Bear in mind that these Lionel cars come without any passengers:
The area of this car that is under the dome is a lounge. I did not modify the seating and only put a couple of passengers in, who are placed squarely in the side windows. Putting in more passengers was surplus to requirements because they would not be seen through the windows on either side of the car. Otherwise, as this is an MKT Texas Special car, I added a totally whimsical "we're not in Kansas any more" theme, based loosely on the fact that poster art was a feature of most long-distance streamliners of the era:
But this is not what I consider a feature car; reverting to the UP dome lounge observation car, which is, I went overboard in populating the observation compartment, mostly with excursion passengers who are on a party/picnic outing:
To do what I considered justice to the prototypical features of this compartment, which include the curved seating and the several wood coffee stands, I had to rummage through several different sets of passenger figures. A few are S scale Artistta figures, which are made out of metal. I pretty much exhausted the entire range of "personalities" that are available in this one section.
However, this is not my most populated feature car. That possibly dubious honor goes to a Lionel 21" aluminum dining car, the Sam Houston from Lionel's Texas Special set. Here I modified the dining compartment from the original and prototypical 36 seater to 30 and put in over 30 diners, drinkers and stewards with the idea of having one of them visible in every window:
My favorite vignette of anything I have built to date is in another car of this train and was created by accident when a female passenger fell over in the course of being glued upright. I decided that she could stay tipped over as well as tipsy:
All this is a long way from the idealized passengers of railroad advertising of the time, which is illustrated by this poster for the GM Train of Tomorrow:
But a certain amount of artistic license was permissible then and I have followed suit.