OK! EXCLUSIVE: Secrets From 'Eclipse' Set — Kristen's Wonky Wig!

The shag cut Kristen Stewart sported to play Joan Jett in The Runaways hadn't yet grown out, so she donned extensions to play Bella! But K-Stew's faux locks weren't cooperative at first.

Kristen's hairpiece went askew, and test audiences noticed, so director David Slade had to redo some scenes!

After a long wait, she got a wig, and those flowing Bella locks stayed put!

"Kristen was a good sport," a source tells OK!.

That wig must have been secure to stay on during all of K-Stew's highly anticipated romantic scenes with Robert Pattinson and Taylor Lautner.

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