What is happerning to Laura in FWWM when shes with Harold and shes talking about BOB and asks him to save the diary when she goes all wierd and her lips go purple and she says 'fire walk with me.....ME!!!'???? That bit always confuses me

BOB's influence. Because of the struggle between Laura and him, where he wants to be inside her but she resists (as Leland described), she's never actually possessed but suffers from his influence. That's why her clothes seem to be 'contaminated' (glasses, what Donna wears in the Pink Room) and the reason of the black lips and yellow teeth, which in the end of the film are seen in Leland...

Yeah, it's almost like she lets BOB in for a couple of seconds, just to show Harold that it's real.

Thanks people, i thought something along the lines but wasnt 100% sure. Its clear to me now

I think in the end all her defenses were broken down. BOB was getting desperate, just the day before that he had exposed himself to Laura when Mike/Phillip Gerard followed them in the car, she was close to the truth, he was close to breaking, the only way to go was to have her by the end of the week or BOB would be in trouble, Laura knew she had no strength left to resist BOB, that is why she knew she was going to die, because there was no way she was gonna let BOB in, she was sinning at that moment, turning Harold into something different than he was for her, and paid the price, too far gone to keep him ouy, Leland also doesn't look this way until he's completely broken down...