The cornerstone of any TV show is its pilot episode. This is the frequently low-rent piece of film that finds its way to the screens of producers and studio executives, and helps get the series green-lit.
Sometimes a show's creators already have a good chunk of funding in place, and the high production value of the pilot means it can be integrated straight into the series as its first episode, but often substantial changes are made to the plot, set and look of things before we ever see a second of it (doubly so in the days before Netflix and made-to-order series).
Unfortunately (for them), this also often includes the cast.
As Gilderoy Lockhart said, fame is a fickle friend. But for some the opportunity for celebrity glory, armfuls of Emmys and the common people getting your kisser tattooed on every appendage is sadly denied before the ink on the contract has dried.
These pilot actors frequently part from productions before the bulk of filming is underway, and recasting sessions help find the face destined to be burned into our retinas for the next five to ten years.
Yes, a number of famous performances from a whole host of our favourite TV shows could have been remarkably different and, like the Peking butterfly's wings causing rain in Central Park, who knows just how different our lives and loves would be had the original actor not been ousted at the eleventh hour!
10. Rachael MacFarlane - Meg Griffin (Family Guy)
First up, we have something of a tumbling cascade of actresses, beginning with Rachael MacFarlane as Meg in the test pilot episode of Family Guy.
Rachael's stay was short-lived, designed to fill a gap on a tight budget while providing enough character to give the studios a taste of what was to come. Nonetheless, she set the template for the sort of voice actress the show wanted to fill Meg's pink beanie: the voice of someone hot, sociable and the complete antithesis of Meg herself.
Following on from Rachael, Lacey Chabert (Mean Girls' very own Gretchen Wieners) took on the role in the official pilot, and carried the character throughout season one. Then followed the iconic Mila Kunis, from season two onwards, and since then Family Guy has had a whale of a time often at her expense.
As she is series creator Seth MacFarlane's sister, it makes sense that Rachael was eventually found another role on Family Guy's sister show, American Dad!, and she has now spent 16 years as regular cast member and the Smith family's rebellious daughter, Hayley.