He's also (to my knowledge) one of the only major Hollywood actors to ever star in a movie filmed entirely in esperanto. I've heard that the pronunciation is rather rough around the edges though I have no way of corroborating that.
> Native Esperanto speakers (Esperanto: denaskuloj [denasˈkuloi̯] or denaskaj esperantistoj [deˈnaskai̯ esperanˈtistoi̯]) are people who have acquired Esperanto as one of their native languages. As of 1996, there were 350 or so attested cases of families with native Esperanto speakers.[1][2] Estimates from associations indicate that there were around 1,000 Esperanto-speaking families, involving perhaps 2,000 children in 2004. ...
> some families have passed Esperanto on to their children over several generations.
My Esperanto teacher told me 'there are always a lot of marriages after Esperanto conventions '. It makes sense it would be the primary shared language of some couples
The Wikipedia article points out 'native speakers have limited opportunity to meet one another except where meetings are specially arranged. For that reason, many parents consider it important to bring their children regularly to Esperanto conventions'.
I'd heard of this movie before and had to check it out - the scene I watched where he was speaking to who I assume is the female lead sounded like an American and an Italian both speaking perfectly passable Esperanto. If I were to nitpick his i's were a little soft - Esperanto i's are canonically pronounced ee - but Esperanto was made to accommodate lots of different accents without losing comprehensibility and it did that here. I actually found his Esperanto easier to follow than the girl's, but that's probably because I learned Esperanto from people speaking it with Canadian accents.
When I watched Incubus I remember him sounding very much like he was trying to speak Italian. My only basis for comparison are some podcasts in Esperanto I've listened to, and completion of the duolingo course (I've forgotten everything).
https://en.wikipedia.org/wiki/Incubus_(1966_film)