In Shannen's case I think that's untrue. Both her and Holly have said that when Shannen would get a Charmed script and if she thought it could be better in places she'd get together with Holly and talk about how they where going to make the scene better. It's about their imagination and how they feel it should be acted out. The diaolgue remains but the actions are there own
Brian Austin Green has said a similar thing about her
But I agree, she does go for roles that she can really kind of show her range. She said in an interview that the reason she did Another Day was because she wasn't sure if she could pull the character off, that she give her the right depth and so she had to do it to see whether she could do it right. I think that's one of her best movies!
She'll do stuff with Brenda she didn't get to do before. I think that's wonderful!
*hugs*
xoxoxo