The guy is James Garner, and the place he wanders around in is Manhattan. If you want to see the city in black and white in the middle '60s, the movie's worth a look for that alone. And notice the the women he becomes a bit involved with in wanderings. Jean Simmons, Suzanne Pleshette, Katherine Ross, and Angela Lasbury (Jessica Fletcher was never like this!).
The movie's a bit disjointed, but it's worth watching for the setting and the performances. The book is better, another of Hunter's big bestsellers before Ed McBain took over, but watch the movie if you get a chance. It shows up on TCM now and then.