You don't want your fingers drifting into the barrel/cylinder gap, unless you want to be rid of them. Here are some tips on a proper defensive grip on a revolver from world-record holder and Team Smith & Wesson shooter Jerry Miculek.
1. The key to a correct revolver grip is the placement of the trigger finger on the trigger. It needs to be square to the face of the trigger. The grip is secondary to the placement of the finger on the trigger. By placing your trigger finger squarely on the trigger and then aligning your grip, you will have better control over the revolver. The trigger finger should index the trigger between the last joint and the tip of the finger.
2. Try to keep the centerline of the barrel aligned below the centerline of the arm. When you discharge the revolver, it will recoil straight back. What I do is get my hand as high as I can on the grip while still allowing the revolver to function. The web of my shooting hand is typically over the backstrap, but still allows the hammer to function properly. Remember to keep your hand up high and the barrel down low.
3. I try to get 70 percent of the control of the revolver using the non-firing hand. When gripping the revolver, I wrap both thumbs over and hold tightly. This allows me to get that control with my non-firing hand while loosening my grip with my dominant hand. One thing you want to remember is consistency of presentation to the target. Also, keep your fingers behind the barrel/cylinder gap at all times.
4. On a J-frame revolver, due to the shortened grip, you need to modify your hold. I do this by overlapping my thumb on the non-shooting hand across the backstrap. When doing this, I'm actually pinching the revolver down with both thumbs, giving me better consistency from shot to shot.