The Running On Empty star plays the mother of Katie Holmes' bipolar character in Touched With Fire and she admits watching her co-star go through the motions of a manic depressive brought back wonderful and terrible memories of her sister's battle.

"My sister struggled with severe bipolar illness and I mean her highs were psychotic highs and her lows were very bad," Christine told WENN. "So I went into this film with a lot of knowledge.

"I read the script and I met with Paul and, since he's bipolar, I thought I had to do this film. This is his story in a way."

Lahti's biggest regret is her sister won't see the film: "She didn't make it. She fought a valiant fight and went through a roller-coaster of hell and she had enough.

"Unfortunately that is not a rare story and not an uncommon story that I'm finding out about bi-polar disorder. As high as you go in the mania, that is how low you will go in the depression; that is the cycle."

But the actress hopes Touched With Fire will bring awareness to the condition, and help those struggling with bipolar disorder.

"Maybe, with this movie, people will not be so ashamed," she added. "Maybe people who suffer from it will be moved just to be able to say, 'I'm not ashamed of this and there's no reason for me to be ashamed'. It's no different than someone having diabetes, in that it's a disease where you have to be diligent about your medication.

"Find the right medication first; that's crucial. And carry on a more productive life. I think if others are more accepting of it, it will be a lot easier for those people to do that. I hope this film shows that. If you're bipolar or you know someone who is bipolar they're probably really creative people. And if they can just stabilise on their medication, man go for it! Go and get a great life for yourself."