Dave Gettleman used to sit on the front porch of the apartment building where he grew up in Boston and just observe. For hours, he said, he would notice how folks walked, talked and behaved when they weren’t even aware anyone was looking. Certainly not some kid on the stoop. “I’m an inveterate people watcher,” he said. “I’d watch people. My father told me that you can learn a lot by doing that, and he was absolutely right.” It’s a skill that he’ll bring with him to Indianapolis this week as he attends the NFL Scouting Combine for the first time as general manager of the Giants. So while other people are busy writing down split times for 40-yard dashes and counting off bench press reps — things Gettleman calls “that height-weight-speed silliness” — he’ll essentially be back on the porch of that triple-decker apartment building, quietly taking mental notes on all of the prospects. “A lot of it is just watching how guys carry themselves and how they interact with other people and how they are on the floor,” he said. “It’s another piece of the puzzle.” There are, Gettleman said, a lot of pieces. In an exclusive interview with Newsday on Friday, the general manager who will have been on the job for less than two months when he arrives in Indianapolis early this week spoke about what he wants to accomplish at this year’s Combine, what he’s looking for in potential future Giants, and why he will not be coming home with a conviction on what he’ll do with the No. 2 overall pick in late April.