I have a feeling that they (the cast, Jensen, Jared, Gen and Osric) uploaded the video to YouTube and then started the timer, got on Tumblr, and waited for the first gif set and reaction posts to pop up.
“oh! there’s the first one!”
“oh look! they really love it!”
“look at how many views it’s gotten in 10 minutes!”
“holy shit there’s too many gifsets to keep track of!”
*accidentally ventures into Destiel and Wincest*
“oh that’s a good idea! we should
hint at that on the next episode and entitle it ‘is this fanfiction or is this canon?’”
In all seriousness, Jensen was probably the most enthused about it.