What I meant was he took immediate action which was uncharacteristic of McClellan. I think he acted untypical of his past performance by taking the initiative and acting on the intel in his procession. I think that is truly uncharacteristic of the man. I would not at all have been surprised that he received the good fortune of finding the document and distrusting the content. That would have been more his style. The fact that he not only took action but immediate action is what I find uncharacteristic in the SO 191 account.
I don't think it's atypical. You must disambiguate two different situations: fluid field situations of maneouver, and static "siege" situations with heavily entrenched opposition.
In the open field McClellan was pretty bold and aggressive, although not to the point of rashness. Often his aggressive nature was not matched by his subordinates, or once (Williamsburg) overridden by a couple of hopelessly rash division commanders who knocked their heads against fortifications and lost.
When McClellan comes up against a heavily entrenched enemy (North Va, winter of 61-2, Yorktown, Richmond) he looks to turn the enemy out of position if possible, and if not he conducts deliberate siege operations.
In Maryland the capture of SO191 merely confirmed what his other int indicators were telling him. He'd sent 9th Corps to attack South Mountain before he was in possession of SO191, because he'd pushed cavalry out that way and they'd told him the enemy was holding the gaps. This was contradicted by what Halleck and Curtain were telling him.
Halleck was telling McClellan the whole rebel invasion was a diversion, and the rebels would seize Washington with another army. Curtain was telling McClellan the rebels were already marching into Pennsylvania, and the army needed to come up there (McClellan dispatched some cavalry to Gettysburg to check this out).
Bursting through the South Mountain gaps McClellan's int was that the rebel army was in three pieces - one at Sharpsburg, one retreating to Williamsport and another at Harper's Ferry. He resolved to strike the rebel centre at Sharpsburg with his entire force. His plan of action was to assault the enemy left and right simultaneously with 1st and 9th Corps to cut off the roads to the north and the fords. Thus on the late morning of the
16th McClellan ordered both Burnside and Hooker forward, but Burnside didn't move. He continues with the plan the next day, and Antietam results.
Of course after the rebel retreat his intention is to hold the Potomac crossings at Shepherdstown and Williamsport with detachments and immediately move his main body to Winchester to envelop Lee, but Halleck (and Lincoln) object to the plan for exposing Washington and operations cease for a month.