For a long time there was debate among historians over whether people were tied or nailed to the cross. It is now mainly thought that it was both, because if you just fasten them with rope their arms and legs shrink as the circulation declines and then slip out, while nails can and will pull through.

Also after a while, hanging from the ropes becomes painful, and the victim finds it hard work to breath, they could however, and did attempt to ease this by moving, which they could only do by using the nails to raise their weight, which is also painful, but it prolongs the suffering by helping breathing. The constant need to try and find ease between the pain of the ropes and the nails therefore kept the victim constantly squirming, which was regarded by the Romans as the main horror of that form of death. After a while, fluid builds up around the lungs and this could stop breathing, causing a quicker death. So it is thought that the practice of pushing a spear point into the side under the ribs, to let off the fluid, and thus further prolong suffering, grew up. By giving hydration in the form of water made unpleasant with vinegar, the victim could be kept squirming for three or four days.

I do not know if telling them this will make any difference, but you could try it.

( Some people have speculated that the idea that Jesus was lowered from the cross early after barely three days , much sooner than usual, thanks to Joseph bribing the guard. Was a sign that maybe he was not truly dead when brought down. Which could explain a lot. )