I thought THIS was interesting. Something I’d never thought possible, but with the long history of stage make up, it all makes sense. In a way, it’s too bad we still don’t do it.
I read it here
Keira Knightley had bosoms and cleavage painted on in the Piirates film.  She’s very flat chested!  Click on the link to see the photo comparisons!

.....the film-studio art of “bosom make-up”. It is a skill from Hollywood’s golden age that fell out of favour with the advent of silicone enhancement.
Miss Knightley said: “They painted my t**s on me for the films, which is extraordinary because it’s kind of a dying art form - in the past, they used to have whole sections of the studios devoted to bosom make-up.

“And I loved it, completely loved it. Because it was the first time in my life I had big t**s, and I didn’t even need surgery.’

She said the process of creating the cleavage took 45 minutes every morning before filming started.

A make-up artist would apply a slightly darker shade of base make-up between Miss Knightley’s own breasts to create a shadow to increase the ‘cleavage effect’.