I'm really amazed USA was able to get such a strong cast together with really established actors.
At the time I don't think most of the cast were really that established.
Adam Baldwin had a famous last name and was known for his role in Full Metal Jacket but that was about it.
Nathan Fillion and Alan Tudyk had also had some short runs on TV and memorable bit parts but neither had been main characters in anything big.
Joss Whedon always gets praised for his writing and the shows he creates but either he has been really lucky or he is a great caster too.
Buffy, Angel & Firefly had some a lot of unknowns step in and do really great work.