Roger Stone is shown in the documentary, A Storm Foretold, by filmmakers Christoffer Guldbrandsen and Frederik Marbell, bragging about how he has manipulated Trump for decades. Via The Daily Beast:
“I have a 40-year record of being able to convince the big man to do what’s in his best interest -- he’s not easy to deal with," Stone says in the film. "It’s complicated. He resents any implication that he is handled or managed or directed.”
Stone says he plants ideas in Trump's head by making him think they're his own, with a dollop of flattery.
“You have to say, ‘Remember that night when we were in Buffalo and you gave that speech, and God, it had to be 10,000 people, the biggest crowd they’d ever seen, and you said XYZ, and the place went crazy, remember that?'" Stone says. "I don’t know where you came up with that line, but it’s one of the best things.’”
Stone then says Trump often might say he'll use that line again, and adds that he's used that tactic almost as long as he's been advising the ex-president.
Stone said it "doesn't matter" that Trump never said it. “It’s time-consuming, but it works. I did it for 30 years,” he told the filmmakers.
No, we don't have the quoted clip. But here's another one from the same film.