Plastic people have a terrible sense of balance. I have had a few 1:48 scale people stand on their own, with a little sanding of the soles of their shoes, so that they balance properly.
For making people stand there are several methods.
At the craft stores they sell a sticky clay, some of that on the feet of your people could make them stand.
At the craft stores they also sell "glue dots". That the same sticky stuff that the credit card companies use to stick a credit card to the form when they mail it too you. Putting that stuff on the bottom of the feet of your people could make them stand.
For making people stand on surfaces that aren't hard and solid, you could drill a small hole through the bottom of their foot, and up into their leg, insert a small pin, with the head snipped off, and stick them into the surface. This is good for lawns, dirt areas, fields and such.
Some folks glue their people to clear plastic disks (bases). and then stand them where they want them. Since the disk is clear, they have this assumption that folks won't notice it. But we, the viewers , do notice this.