The movement has to be subtle or it will scare the snake, lol. And rodents don't volunteer to be eaten in the wild either, so don't make it appear that the rodent is approaching the snake. Make it seem like the rodent is just cluelessly strolling too close to where the snake is hiding -so the snake notices & gets the scent & warmth- but without going toward the snake. Go past & slightly away to elicit a slight chase, & so the snake feels brave.