You're probably going to need some kind of support, even if it is a prop that the body is holding, like an umbrella touching the ground to make a third 'leg' which would create a tripod base.

People themselves wouldn't be able to stand up if it weren't for the balance that their brain and muscle control are giving them.

Other than the tripod via a prop, two other things that would help would be weighting the feet with bags of sand or something fairly heavy, and/or attaching the feet to a broader base, like a board, that was heavy enough to provide a larger base and some weight.

But even just weighting the feet wouldn't be very stable unless the feet were fairly large.

Sue