harry potter - If the Imperius Curse gives you a blank look, why was there doubt over whether someone was under it?

In the books, many of Voldemort's Death Eaters, such as Lucius Malfoy, avoided imprisonment after the first Wizarding War by claiming that they were acting under the influence of the Imperius Curse. The film version of Goblet of Fire has Mad Eye Moody succinctly explaining the dilemma.

"Scores of witches and wizards have claimed that they only did You-Know-Who's bidding under the influence of the Imperius Curse. But here's the rub: how do we sort out the liars?"

However, this makes no sense in the film because those under the Imperius Curse have a blank look with obviously wrong eyes.

And in Deathly Hallows, Part I, a goblin has a dopey, blank face after being under the Imperius Curse.

So if people under the Imperius Curse have a blank look, why is there doubt that they were under it?

I'd like an answer that respects film continuity (rather than just being "the films are terrible").

Answer

As we are focusing just on the movies here, the answer is obvious.

In-universe.

The 2 people were not under the imperious curse. That's just an assumption the viewer jumps to having seen the unforgivables performed earlier.

HARRY: No stop! He's bewitched Cedric. They struggle.

CEDRIC: Get off me!

HARRY: He's bewitched!

Note that it doesn't imply or mention the Imperious curse at all. I'm not sure what was used on the Goblin but its possible it was Confundus charm (like in the books). In fact the only 100% guaranteed use of Imperious I remember in the films is with the spider in class, (and that spider looked exactly the same: perfectly normal). So as an In-universe answer no humans were under Imperious curse in the film at all: they were all probably under a different curse. NOTE: this explanation only works for the films but the question specifically limits to just the films.

Out of Universe

There had to be a way to show the viewer the person was under someone else's control. The eyes are the easiest way to do this as others have pointed out already.

Conclusion

I see no evidence in the films to suggest the Imperious Curse changes some-ones physical appearance. Therefor while its implied to the viewer after the class demo, it was never once confirmed other than the viewer assuming that's the curse controlling them, (we never see the curse used on those with milky eyes: so we don't know for sure what curse was used). If there is only truly the spider which we see onscreen then its safe to assume that the Imperious Cure doesn't create a blank look or milky eyes: and anyone under that curse looks perfectly normal. The books are harder to explain away though.

Its been a while since I've seen the films so I could be mistaken on some details please feel free to point them out.