"Je Suis Charlie seems like an absolute no-brainer," Hanks told ET, after showing his support via Instagram post.
"The pen is mightier than the sword and I think it's a wonderful symbol of free speech," Mirren said, wearing a pen pinned to her dress.
"That's the insanity of the world, isn't it?" Rosamund Pike told ET. "The extremities of life -- it all hurdles at us. That's why we feel privileged and we have to enjoy every moment of it."
The surviving staff members of Charlie Hebdo held an emotional press conference in which they revealed the cover, which features a depiction of the prophet Muhammad. The two gunmen, brothers Cherif and Said Kouachi, launched the attack last Wednesday, saying that they wanted to avenge Muhammad for Charlie Hebdo's satire of him. CNN's Brian Stelter says journalists are debating whether or not to show the cover.
Former editor of The Onion, Joe Randazzo, opened up about what the movement means to other satirical outlets.
"You always offend somebody no matter what the joke is when you're working for The Onion," Randazzo said. "If we start to worry about violence being a retribution for anything we say under the context of satire then it really will erode at our concept of free speech."
Normally Charlie Hebdo prints about 60,000 copies per issue, but Wednesday's edition will have 3 million magazines printed.