This is not a topic for the pros and cons of whether or not to put interiors into your buildings ... this is just to showcase peoples work and swap tips on how to source/modify parts for interiors.
I model in HO ... here is an interior I've just completed .... a young woman (circa 2005) is putting her suitcase back on top of the wardrobe after returning from a holiday.
OK, here's some I did for my Station building, which is here:
All these are built as separate modules which then slot into the building.
First, the interior of the booking hall:
Then the booking office:
The Stationmaster's office:
The ladies' waiting room
and finally, one of the ladies' loos:
There you go...
Al.