From an interview in the Orange County Register, 1997:
"I'm actually aroused by the silver in a way - it's a peculiar fascination." Firth is speaking first person for his character. Such is the Darcy-like intensity of Firth, who smokes furiously. His hair has lightened from Darcy dark to its normal chestnut.

"In Nostromo I make love to my wife in the silver mine - a real mixed message and mixed motivation. Ennio Morricone, who wrote the music, wrote a love theme for me and my wife. Later whenever I look at the silver it becomes the same love theme.

It's very clear what happens to Charles Gould - he becomes a man who loses his soul. Still, Conrad never allows his characters to be simple. Gould wants the silver to civilize what is regarded as an uncivilized country. He wants all those turn-of-the-century ideals - beauty, order, truth - to come from the silver. He sticks with that even when it's quite clear that he's turning into a monster".

For Firth and his fellow actors, Nostromo also was a dangerous kind of chaos. "You'd be sitting on a horse that wasn't really trained in front of 50 to 100 other horses and carriages on a dirt street in a shantytown with the camera miles away and a huge crowd and a language barrier and explosives going off.

They gave me a quite uncontrollable horse the first day, a mustang or something, and I was thrown for the first time in my life - and I pride myself on being quite good...

And there's a scene where I'm being garroted - do you know what garroting is? There's a metal collar to strangle me around my neck and someone yelled an instruction which I didn't understand. My hands are tied behind my back and I don't speak Spanish to tell them I'm REALLY being strangled and to stop. And yet we finished on schedule and it looks great."

And Firth lived to tell the tale of emotionally constricted Gould, who remains closed-hearted to the end.
[Orange County Register, January 2, 1997]