In Becoming Part 1, when Willow and Buffy go to the library to tell Giles that there is a cure for Angel's humanity, watch Willow closely. It's very obvious that she is trying hard not to laugh at something and failing.
I noticed this too, I thought her expression was really out of place and I couldn't read it, but now you've said that I realise it. They should have re-shot that scene, but I suppose all your attention's meant to be on Buffy, but when you've seen it loads, you wander.
That isn't the only spot Allison holds back. In "Never Kill a Boy on the First Date" check out the viewing room scene with Owen, Xander, and Willow, when they realize the dead body in the autopsy room is now a Vampire. Allison clings to Nicholas Brenden's arm trying to be afraid but it doesn't work. In all fairness, non of the buffy cast were great actors the first season or two.
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