The 'Still Alice' star is working with Everytown for Gun Safety and has launched Everytown Creative Council, which unites creative people who are in support of the prevention of gun violence.
Recalling how her daughter, Liv, now 13, reacted to the Sandy Hook school shooting in 2012, she said: "I remember my daughter came to work with me that day. I was doing a movie in Queens and when we got into the van, I actually said to the driver, 'Turn the radio off.' She was young so the whole day, I kept the TV and the radio off.
"She had her phone with her and she said, 'Mommy, did some kids get shot?' I was like, 'I've had it.' I said to my husband, [director Bart Freundlich], 'I've got to do something. This is the one thing that I need to say something about. This is my responsibility as a parent."
The 54-year-old actress has already signed up 79 celebrities - including Steve Carrell, Reese Witherspoon and Ellen DeGeneres - and hopes she can recruit even more to join the cause.
She added to PEOPLE magazine: "I was going through my address book alphabetically. That's all I did. 'Please, are you interested,' and the response has been overwhelming. I'm so, so thrilled. Everybody feels this way about this issue.
"I feel like with gun safety, there have been many people who feel threatened by saying something but I don't think I'm saying anything that's unreasonable. With all of these rights that we have in our Constitution come a great deal of responsibility, and I think the founding fathers would agree with me."