Complex sentence: Unsatisfied with the preparations for the shoot, Jack is suddenly enthralled with Sarah, whom he had never noticed before, and asks her to replace the other model, which she promptly agrees to with a knowing smile.
Split into simpler sentences: Unsatisfied with the preparations for the shoot, Jack is suddenly enthralled with Sarah, whom he had never noticed before. He asks her to replace the other model, to which she promptly agrees to with a knowing smile.