Speaking of genetics, my daughter's father tended to be poker faced (He's less so now, as he's gotten older). I laid it to the fact that his mother had died when he was three years old and he was reared by a series of relatives. I thought that he had developed a noncommittal expression in self defense.

However, by the time our daughter was four years old it alarmed me that her face resembled his inexpressiveness. She had not lived with her father since she was a baby- we had divorced early on- so she wasn't consciously copying him.

There was no way I wanted her to live with that face so I bought a bunch of small inexpensive toys I knew she'd like and then one by one I gave them to her, telling her things like: Now say, For me?! Thank you! How pretty!

You get the idea. I did this a couple of days in a row. She still remembers the drill.

I don't know if it is what did the trick but you couldn't find a more expressive, demonstrative person anywhere!