Star Trek: 10 Things You Didn't Know About Julian Bashir

Yes, he killed everyone on the Yukon and tried to create a supernova that would have destroyed the entire Bajoran star system — Deep Space 9 and the fleet along with it — but at least the changeling masquerading as Bashir had the otherwise good manners to bring snacks to his work-weary 'colleagues'. "A gentleman and a scholar," in O'Brien's words, none the wiser that his best friend had been replaced by polymorphic goo (no offence, Odo).

It was around the point of the sandwiches in In Purgatory's Shadow that we, the audience, found out that Bashir had been in a Dominion internment centre on an asteroid somewhere in the Gamma Quadrant since at least before the uniform change. In a manner similar to Bashir's genetic status, the changeling 'big reveal' wasn't planned much, if at all, in advance. As Robert Hewitt Wolfe stated in The Star Trek: Deep Space Nine Companion:

We thought, 'Okay, we've got prisoners of the Dominion, but we need a surprise. Something we don't expect. And suddenly we learn that Bashir's a prisoner too. He's been replaced on the station.'

Naturally, the news came as quite the surprise to Alexander Siddig, too, who noted in the DS9 Companion:

Apparently, I'd been a changeling for the previous three or four shows […] But I didn't know until the last minute. […] What a shock!

For reference, see the look on Changeling Bashir's face when the Defiant swoops in in By Inferno's Light.