The actor was on Brian Stelter's "Reliable Sources" Sunday morning as a special guest, where he was asked about the impeachment inquiry opened against DT this past week ... and where he thinks things will go from here. Similar to our question, of course.

You'll recall ... we asked Bob just Saturday about this issue, and he somewhat surprised us by saying he'd actually rather have Mike Pence in office ... if it meant getting Donnie out.

Welp, RDN expanded on that a little with the CNN host ... making a pretty serious claim that he believes POTUS demonstrates "crazy" behavior, and he seemed to mean it literally. Stelter asked him if he was saying "crazy" in a medical sense, and Bob said possibly.

Then came the f-bombs loud and clear on cable TV, which Stelter joked wouldn't quite get them in trouble with the FCC ... but he did frown on the foul language. He probably shouldn't have been surprised though ... De Niro used the f-word at the Tonys, after all.

Stelter had asked how De Niro feels when Fox News pundits attack him for criticizing Trump with such ferocity, and Bob fired back with a simple, "F*** 'em." Twice, no less! BS asked why RDN uses that type of language, and he had a pretty interesting answer.