Oddly enough, it's not even that far off from what really happened:

What happened was that director Peter Jackson was watching Noble munch on these delicacies as part of his rehearsals, and for the sake of continuity, Noble had to make sure that there’s a sort of rhythm to the way he ate the food. Jackson got notably excited with the cherry tomato bit, and that was how the scene made its appearance.

“So I’m sitting there thinking, ‘ok…with this food I’m going to go 1, 2, 3, 4, 5…1, 2, 3, 4, 5…’, so I could repeat it, the timing would be perfect. Jackson must’ve been watching through the monitors and he saw me squirt something out of the side of my mouth. He got terribly excited! He ran on to the set and said, ‘mate, what you just did there?” ” What was that, mate?” “When you squirted…can you do that?’ I said ‘yes’. ‘Can you go like hmff?’ I go, ‘yes’. He got terribly excited and waddled away again. That’s how that actually came about, because he was watching me rehearse to try and get my continuity right,” said Noble.

https://www.thegamer.com/lets-talk-about-that-infamous-lord-of-the-ring-cherry-tomato-scene-with-denethor/