EXCLUSIVE: Tom Selleck Previews 'Blue Bloods' Season Finale 'Twist': 'It Changes Frank's Life'

Blue Bloods' season finale airs Friday night, and after seven seasons, the CBS drama is still full of surprises.

ET caught up with Tom Selleck over the phone on Thursday, who previewed the shocking episode -- and noted that some fans will have caught that the show "planted some seeds" early on as to what twists might be ahead.

WATCH: Kathie Lee Gifford and Tom Selleck Recreate Their 'Live!' Kiss From 20 Years Ago

"We have kind of a cumulative narrative on the show. Each experience [the characters] have informs them about the future," the 72-year-old actor shared. "I think for Frank, as much as he's had an adversarial relationship with the mayor, played so well by David Ramsey, when push came to shove, they always found a way there. What has been happening in the show [recently], and I think planted some seeds for the audience, is that there's been something different."

"We're leading up to a pretty serious confrontation," he added. "I don't want to tell the audience how it turns out, but there's a very nice twist in the show."

"I think it obviously changes Frank's life," Selleck confessed, before noting that he "honestly never knows" what the Blue Bloods writers have in store. "I've gotten very comfortable with the notion that the writers come up with ideas when they formulate a story. They may come to us with notes, but I don't know where the character [will end up]."

The actor does know, however, that the finale's twist will be a "very big deal" for his character.

EXCLUSIVE: Tom Selleck Has Never Used Email, Wants to Play Himself in a 'Magnum P.I.' Remake

"I think it will give us some very interesting stuff," he said. "I don't think the consequences have ever been this broad...I don't know where they'll go with it, but it gives
our writers material for the character, who really has some serious questions of conscience, based on really pushing the envelope in this case."

"I know [the writers] are mulling over ideas, and they call me saying, 'What do you think?'" he revealed. "But I have a hunch the Reagans don't stand down...We are far from out of ideas. There are lots of characters to grow and change, so what happens [in the finale] is important, but not as important as how it lands."

"It's how what we just saw will affect the characters," he explained.

Currently wrapping up its seventh season, and renewed for season eight, Blue Bloods definitely has room to continue the story, but Selleck says he still can't believe the series' success.

EXCLUSIVE: The 'Blue Bloods' Gang Have the Most Uncomfortable Family Dinner Ever

"It is [unbelievable]. I never thought I'd be this lucky twice, but when we get to 163, I will have tied Magnum, P.I., just nine shows in. So that's pretty neat," he revealed. "[I can't believe] I'm lucky enough to have a show with this kind of viability. I think it's viable in its eighth year, and it's still growing, really, in many senses."

"We're not out of ideas, and I think the show has incredible potential," Selleck added. "I know all of our deals are up at the end of eight seasons, and I don't want to jinx anything, but I think CBS would be crazy not to continue it."

"I certainly know that my character, and the other characters, have plenty of room, that we, as actors, will stay challenged by the show," he admitted. "If we stay challenged, the show has... I can't tell you how long."

The season finale of Blue Bloods airs Friday at 10 p.m. ET/PT on CBS.

EXCLUSIVE: Donnie Wahlberg on Directing Tom Selleck in 'Blue Bloods'