Spotted: Bernie Sanders, walking down Sixth Avenue in Manhattan. Swinging hands with his wife, Jane. Pointing to buildings. Looking at the sidewalk.

And talking as little as possible to people. Not asking for a single vote.

He did consent to the occasional selfie — “Sure,” “O.K.” — but his chattiest interaction came with a man who stopped short at the sight of the Democratic presidential contender and blurted out his name.

“How are you?” Mr. Sanders asked.

“I’m good, but I’m voting for Trump,” the man said.

Mr. Sanders walked on a few steps, turned back and opened his mouth as if to reply — but not a word came out.

For a candidate who has inspired the most impassioned followers since Barack Obama in 2008, Mr. Sanders is surprisingly impersonal, even uninterested, in one-on-one exchanges — the sort of momentary encounters in which a candidate can show warmth and humility by gripping every open palm.