A way to potentially nab the rat would seem like it would be pretty important for a crew like Costellos don't ya think. So when Costigan just says he ain't doing it and leaves....it's all like "do wut your told pls" and thats it. Costello didn't seem to really care that Coztigan straight up ignored very important procedures.
shareI could be wrong - but it appears that Costigan does do it - he just doesn't stay. He fills it out at the bar next to Fitzy - then gives it to Delahunt - the only order he disobeys is that he doesn't stay.
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Not only did he provide the information, but he should have had no fear in doing so.
Remember? The reason they chose him to go under cover in the first place was because he already had a history of violence, family in the mob, and they put him through a short run through jail...
Anything they found on his SSN would have been what he wanted them to see.
He didn't want to stick around because he was already a nervous wreck about them finding out he was a cop. He avoided confrontation as much as he could.
It was a question mark for Frank Costello for sure.
But as Costello said later in the restaurant that he couldn't just kill everyone this time. He needed to be sure, and at the end of the movie, Costello trusted Costigan more than anyone and was bringing him up like another "son" of his.
Fitzy seemed to be the most suspicious.
I think he was going to get caught soon. Something had to break.