Putting requires a great deal of focus. All distractions should be eliminated if possible. This would include talking. A moment of silence is proper etiquette when a fellow golfer is putting.
Movement can also be distracting to the golfer. Stand behind the putting golfer, out of their peripheral vision. If you need to observe the roll of the putt, remain behind the putting golfer until they strike the ball. Once the ball has left the face of the putter, you may move to get a better view.
To stand and watch the roll from the other side of the hole, directly across from the putting golfer, would be a serious breach of etiquette.