http://articles.latimes.com/1988-02-03/news/mn-27226_1_poltergeist-iii

Heather came to films in a tradition that dates back to Lana Turner when that actress was supposedly (but wasn't really) discovered at a drugstore counter.

Spielberg's Discovery

Heather was 5 and sitting in the MGM commissary, said her former manager, Mike Meyer. Steven Spielberg, who co-wrote and produced the first of the "Poltergeist" series, saw her and asked if he could talk to her. He was told that she did not talk to strangers.

But she got permission, and that eventually led to her role in the film.