alt Hacker News> Natalie Portman...you could easily tell she wasn't a dancer.
Which is interesting, because from what I can tell she studied ballet from a young age, which potentially puts a hole in your theory. Unless you're only taking about professional dancers who started young versus professional dancers who started late, rather than any (i.e. non-professional) dancer.
Natalie Portman "took ballet as a kid", which is probably similar to like 50% of women in the US. From what I found online, it says "she took ballet from age 4 to 12", but there is a collosal difference between someone who engaged in "professional track" training and someone who engaged it as an after-school hobby.
The ballet world even has a name for small neighborhood dance schools, a "Dolly Dinkle" school (it's a little bit of a knock, but not much, as most professional dancers started at one of these places before moving on to professional training).
But for contrast, take a look at the 12 year olds at the Vaganova Ballet Academy. At that point they've only been at the academy 2 or 3 years, and while they have some "child mannerisms" in their dancing, they all hold and carry themselves like professional dancers.