Talk:The Times They Are A Changeling/@comment-26423071-20160821015125/@comment-4531340-20160821101424

I think the key difference is as follows: in this case, Thorax was vouched for by Spike, Twilight's #1 assistant and obviously someone she trusts very much - so much so that she felt that she could drop her guard in front of the changeling, even though she had no prior experience with him.

In the case of Trixie, she had nopony to vouch for her, and what's more, Twilight had had previous (and somewhat negative) experience with Trixie, so it's natural she'd be more wary (to say nothing of the fact that, honestly, I think it was more the fact that Twilight was wary of Trixie befriending Starlight so suddenly than anything else, considering the history between Twilight and Trixie).