EXCLUSIVE: Jim Parsons Says 'Young Sheldon' Star Iain Armitage 'Popped Immediately' -- Watch the Trailer!

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Jim Parsons
is just as excited about Young Sheldon as we are!

The Big Bang Theory star serves as the narrator for CBS' anticipated fall comedy series, which tells the story of a 9-year-old Sheldon Cooper (Iain Armitage) as he grows up in East Texas, alongside his parents and two siblings. ET caught up with Parsons on Wednesday, where he gushed about his new show, which he will also serve as executive producer.

RELATED: First Look at 'Big Bang Theory' Spinoff 'Young Sheldon'

"Iain is just really special," Parsons exclusively told ET's Leanne Aguilera of the young star, ahead of CBS' Upfront presentation at Carnegie Hall in New York City.

The 44-year-old couldn't help but marvel over his mini-me, praising the young Big Little Lies actor for having impressive acting chops.

"We saw lots and lots of tapes and we saw lots of auditions and there's something about Iain that just popped immediately," Parsons gushed. "There was no denying it as soon as you saw his face and he handles the language well and he memorizes easily, and the biggest thing of all is he's just an excellent actor. I don't know how someone so young acts so well. I didn't when I was his age, but he does."

Parsons did offer Armitage some advice, having played adult Sheldon for a decade on the flagship Big Bang Theory show.

RELATED: 'The Big Bang Theory' Spinoff Is Officially Happening

"We sort of buddied together and sometimes it would range from stuff like, 'Say this line a little differently because Sheldon would think of it differently.' 'Don't smile,'" Armitage adorably revealed to ET.

"He really didn't need very much help," Parsons added. "In some ways, he's too good of an actor, at first blush, with the material. He's listening too well. He's being too empathetic, which is really good acting but not in this case. There were just a couple things you had to curb, like, 'I know you want to laugh and smile and be nice to people [but] don't do that anymore!'"

Watch the five-minute trailer below.

Young Sheldon
premieres after The Big Bang Theory season 11 return on Monday, Sept. 25 at 8:30 p.m. ET/PT before debuting in its regular time slot Thursday, Nov. 2 at 8:30 p.m. ET/PT on CBS.