Jason would be played by Ron Livingston. I have no idea if Ron's personality would match Jason's personality, but every time we go out to dinner someone always says "Did anyone ever tell you that you look like the office space guy?" so Ron wins by default.
Emily, my best friend since kindergarten would be played by Keri Russell. First, because Keri has crazy curly hair like Em does, second because Emily loves her and you should be played by someone you love, and third because Keri is really good at portraying very down to earth characters who come off as being completely real, and Emily is the most down to earth completely real person I know!
Jeff-f-f-f-f would be played by Paul Reiser. Not old Paul Reiser from now, but like if I could get a time machine and get younger Paul Reiser then that would be Jeff. They kind of look alike, a little, but every time I would watch Mad About You I would think that their joke delivery and sense of humor were eerily similar.
Sara would probably be played by Kate Beckinsale, because Kate is short, has dark hair and is cute. But, to play Sara she'd have to drop the accent.
Scott would be played by Neil Patrick Harris. Ok sure, Neil is way older than Scott, but he's got amazing comedic timing and could easily pull off Scott's deadpan humor. Plus, he's got just enough geek in him to be a good version of Scott.
Momwich would have to be played by Sally Field I think. I'm pretty sure that Sally is older than Momwich, but she's super classy and versitile enough to show the transition from shy homebody Momwich to the squeeing fangirl Claymate Momwich. Plus, Momwich should TOTALLY be played by someone who's won an Oscar.
I know this next guy isn't an actor, but I think Doug would have to be played by Steve Jobs. This is mostly because Doug is a walking encyclopedia, and Steve is kind of an uber-geek. I think that's pretty much everyone I have for right now. If I come up with any more later, I'll add them.