NASHUA, N.H.—Donald Trump made his decision to start skewering Sen. Ted Cruz as his private jet was approaching here earlier this month.

“Ted is hanging around the top too long,” the Republican presidential front-runner announced on the plane, according to his campaign manager. “Time to take him down.”

Mr. Trump’s airborne verdict to strike at his closest GOP rival and a look at other decisions like it reveal a truth behind his famously pointed attacks: Mr. Trump, not his staff or consultants, personally drives them, and they are both calculated and improvised to adapt to news and polls, with little research or extensive prep work.

Mr. Trump proceeded to question whether Mr. Cruz’s Canadian birth disqualified him. A week later, he tore into the Texas senator about a loan he took from Goldman Sachs to finance his political career and about his notoriety as a Senate “nasty guy.” The onslaught seemed to stall Mr. Cruz’s rise in Iowa, where polls show Mr. Trump holding an advantage.

In a repeated pattern, Mr. Trump has fired personal attacks at rivals when they emerge as a challenge. While his attacks and policy pronouncements often appear to be off-the-cuff, hours spent interviewing Mr. Trump and watching him behind the scenes show how he plots them, most often alone in his jet as he flies to early primary states.