It is easy if there is ducting in place. Of course, the ducting has to have room, and not be blocked, but if that is the case, then it is quick.
First they pull through some "microducting". This looks like a cable, but it is really hollow tubing specifically for fibre. Because it is just plastic tubing, this can be pulled & pushed with some force.
However, the fibre itself is a little more fragile. Instead of pulling the fibre, they use a process known as "blown fibre". There they get an air compressor to blow fibre through the microducting.
Take a look in the pages around here
If there's no ducting available, then there's no option other than to dig, or go overhead.