The big deal is that her agent raped the budget to negotiate her spare time.
So she's out of time and smallville is out of money.
She can have more time, shit happens, but at this late stage in the game all of Smallvilles money is completely accounted for...
Unless she's working for gratis or Tom (or some one else) is paying her out of their own pocket.
The 200th episode of Cheers was just the cast on folding chairs telling stories divided by clips about the show and about the set and cast. Shelly long just arrived uninvited and unexpected, asked for a deckchair and sat down on stage with the rest of paid actors.