1 {scene: the Jurassic}
1 Allosaurus: RAAARRRHH!!
1 Scientist Adam: It's an Allosaurus!
2 Scientist Jamie: Don't move. There's a theory that these creatures have such poor eyesight that they can only sense prey when it moves.
3 Scientist Adam: {not moving} A theory?
4 Scientist Jamie: Well if it's not true, we'll be making an important contribution to science.
4 Scientist Adam: And its stomach.

This is fairly common thing amongst animal vision. At least to the extent that moving objects are much easier to spot than still ones. It even applies to humans.

Whether or not it applies to dinosaurs is all a bit speculative...

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2024-08-03 Rerun commentary: It's interesting comparing animal vision to human vision. We and other primates have very good colour discrimination compared to most mammals. Non-primates are essentially what we would call red-green colourblind. I notice this particularly with my dog, Scully. She chases thrown tennis balls and I bought several of different colours. The best ones for me are orange, because I can see them easily on grass. But Scully definitely finds it a lot easier to spot blue balls on the green grass. Orange ones she can't seem to pick up visually at all unless she's very close to it.