Alrighty, I'll get things started.

Olivia Dunham - Julianne Moore
Walter Bishop - Martin Landau
Peter Bishop - James Franco
Phillip Broyles - Samuel L. Jackson
Astrid Farnsworth - Halle Berry
Nina Sharp - Sigourney Weaver
William Bell - Sean Connery

I'm going to do an old fashioned cast. of course, even if they were all alive, this cast would still be impossible for obvious reasons. but this would be cool, nonetheless.

Olivia- Grace Kelly (a young one, obviously)
Walter- Jimmy Stewart (in his later years)
Peter- Cary Grant (in his younger years)
Philip- Sidney Poitier (middle aged)
Astrid- off the top of my head i can't come up with someone from awhile back
Nina- Audrey Hepburn (an older one)
William- Clark Gable (later years)