A Simulacrum is a partially real illusion, which is under your control.
It has all the stats of the creature you are duplicating, but it's friendly to you and it can't learn. It's not actually the creature you duplicated, though.
So if the creature you copied was fiercely independent and highly intelligent, the simulacrum would be fiercely independent, highly intelligent, but also always friendly to you and following your orders to the best of its abilities.
If you told it to "act natural", it would do so, using its high intelligence and independent nature, but it would still obey your orders if you gave any. On the other hand, if you told it to "sit perfectly still and do not move", it would do that as well, as it's under your control.
So it would try to follow the spirit of your orders, since it is friendly to you and wants to help you, but it will be limited in doing so by its abilities and nature. (A dog simulacrum might not understand orders to check out a book from the library. A devil simulacrum might not understand orders to act in good faith if questioned by authorities. Just treat them like real creatures that happen to be your friends and you should be okay.)
So basically you can have either of your options of "engage in small talk for entertainment" and "sit still and do nothing" depending on your preference, as long as the creature is capable of doing it. (The dog simulacrum won't be able to engage in small talk, for example.)
Just be a little careful with it; if you give the instructions to "sit still and do nothing until person X is here" then it will follow those, and if person X is delayed by a week, the simulacrum might dehydrate and die. (They are partially real, after all.)