Effects Artist Jason Johnston on the process behind Jack Jack’s powers:
“We met with the Art Director, Ralph Eggleston, and he was very specific in that Jack-Jack is the source of the fire, he’s not on fire,” Johnston added. “And what that means is that there’ll be no smoke and no embers ever coming off of him, but things that he has lit on fire can have them.”
Another crucial element: “We also don’t want it to be scary.”
Go on…How Pixar Brought Jack Jack’s Powers To Life In Incredibles 2