When it came to The Untouchables, filmmaker Brian De Palma needed an “American gangster actor,” as he says in a new clip from the documentary De Palma. That actor ended up being, of course, Robert De Niro, who played Al Capone in the film about his face off with a federal agent. Earlier this week, Entertainment Weekly debuted a separate clip from the doc in which the titular director dishes on wanting to cast Don Johnson instead of De Niro initially, but here he digs into the casting process and getting everyone on board.

“Bobby” — De Palma calls De Niro Bobby — “takes a long time to decide to do things. You go out to dinner with him, you talk about the script, and it took many, many weeks until he finally said, ‘Yeah I think that we can make this work,’” the director said. De Palma also discusses running line with De Niro until he knew them, and how the actor wore silk underwear while in character.

Watch the clip below.

De Palma directors Noah Baumbach and Jake Paltrow recently told Collider about behind-the-scenes discussions with the Hollywood legend.

"We talked about that beforehand, that we would not have our voices in it and to have this be just Brian talking and Brian talking against his movies and not have anyone else talking about Brian’s movies, that Brian’s interpretation of his stories is enough. So it simplifies it in one way and then lets you keep approaching it from one angle, I think."

De Palma had a limited release in theaters on June 10. Here’s the official plot synopsis:

One of the most talented, influential, and iconoclastic filmmakers of all time, Brian De Palma’s career started in the ‘60s and has included such acclaimed and diverse films as Carrie, Dressed to Kill, Blow Out, Scarface, The Untouchables, Carlito’s Way, and Mission: Impossible. In this lively, illuminating and unexpectedly moving documentary, directors Noah Baumbach and Jake Paltrow engage in a personal and candid discussion with De Palma, exploring not only his life and work but also his singular approach to the craft of filmmaking and his remarkable experiences navigating the film business, from his early days as the bad boy of New Hollywood to his more recent years as a respected veteran of the field. In the end, what emerges is a funny, honest, and incisive portrait of a truly one-of-a-kind artist, and an exhilarating behind-the-scenes look at the last 50 years of the film industry through the eyes of someone who has truly seen it all.