It’s not a movie but Band of Brothers was a 10 episode mini series so it’s not like a long running series.
Several had some level of fame before the show.
Donnie Wahlberg was known as a singer but only had one other small acting role in The Sixth Sense.
David Schwimmer was obviously famous.
Ron Livingston had starred in Office Space but it was a flop and not yet a cult movie.
Scott Grimes had been a working child actor for years.
Jimmy Fallon was on SNL but only had a cameo.
The rest were pretty new in their careers and mostly unknown. They range from giant movie stars to actors with busy careers in British and American TV.