"Hey, leave your ball there. It may help me out." "Sure, go for it."
That brief verbal exchange, which occurs when a ball close to the hole might help out a ball about to be played from off the green results in a two-stroke penalty on the players involved. The reason: such a procedure may be detrimental to the rest of the field wherein players are no helping each other. Rule 15.3a is an intent-based Rule, not outcome based. The penalty is assessed once the players make the agreement even if the balls don't collide. The Rule is also applied even if the two players don't know about the penalty. And, if the players do know that such an agreement is not allowed, they are disqualified for deliberately ignoring the Rule. This is a stroke play situation only. In match play, all the players involved are present to protect their rights.
THE QUICK PEAK
It's both prudent and proper for a player to pick up a ball to identify it if the player can't be positive it's his or her ball. However, whether the ball is lifted or just rolled a bit to check for ID, the ball must be marked first. Simply lifting quickly and putting the ball back in the same spot without marking results in a one-stroke penalty. A similar quick peek is when the ball may be embedded in the general area or sitting on a hidden sprinkler head or an animal hole or track or trail. Now the situation has an interesting variation. If the player knows for sure the ball is, for example, embedded and a free drop under 16.4 is available, marking the ball is not necessary because the ball is not going to be replaced in the same spot; however, if the player is not sure, the ball should be marked first because if the ball is not in a relief situation and them must be replaced, lifting without marking results in a one-stroke penalty. So, mark before lifting.
THE NEAT FREAK
It's sometimes necessary for a player to mark and pick up his or her ball when it interferes with another player's ball. For example, if two balls end up side by side just off the green, one player may request the other to mark and lift the interfering ball. The key point in this situation is that the lifted ball may not be cleaned before it is replaced. Cleaning the ball adds one penalty stroke (Rule 15.3b).
Related to the neat freak, is the too helpful player who assumes he or she should mark, and lift his or her ball because it lies close to another player's ball. Unless the ball is on the green, a player can't lift the ball in play in this situation unless requested. Doing so is one penalty stroke (Rule 15.3b).
With your ball on or off the green, ball marks or other damage on the green may be repaired. However, having a putter in your hand with your ball off the green but close enough to putt restricts what you can do with the line of play. That ball mark or old divot or just puffed up grass short of the green may not be altered. Play the course as you find it. Leave the ground as it is. Otherwise, Rule 8.l1 gives you the general penalty of one stroke.
PLAY WELL; NO PENALTIES!