No layout will be completed without people and animals. On the market there are lots of manifacturers selling people, suitable for every need and epoc. The cheapest solution is to buy unpainted figures: we'll paint them according our needs. Generally we would add people even into the rolling stocks: passengers cars and locomotives cannot be empty! We can find lots of sat figures for this purposes.
And don't forget to add some animals... expecially if we have some mountains or country settings.