The mystery movie, based on Nancy Springer's books, centres upon the teenage sister of an already-famous Sherlock Holmes, began shooting last year (19) and is released on Netflix on Wednesday (23Sep20).
Helena thinks that if the movie were produced now, it might have has a different lead due to Hollywood's focus on racial diversity following the Black Lives Matter protests over the spring and summer.
"Everyone's desperate to talk about race these days, but it's a good feminist movie," "The Crown" star tells British newspaper The Times. "If we did it this year we'd have to make Millie black. I'm sure somebody would object to me saying that, but that's true. But it's a very democratic and radical film anyway."
Bosses at the Academy of Motion Picture Arts and Sciences recently announced new guidelines for Best Picture nominees at the Oscars, stating that one main character or 30 per cent of smaller ones must be played by actors of colour.
Helena loved working with Millie on the new movie, on which the 16-year-old is also a producer.
"I thought I was going to be her old mentor, but then I realised very quickly that she was much older than me inside, much more sophisticated," she explains.