In Airplane! his deadpan delivery contrasted with the continual absurdity surrounding him. In the film when asked, "Surely you can't be serious?", he responds with a curt, "I am serious. And don't call me Shirley." He later reflected in several interviews on the significance of the comedic line: "I thought it was amusing, but it never occurred to me that it was going to become a trademark. It's such a surprise...the thing comes out, people say, 'What did he say?!'". Nielsen also stated that he was "...pleased and honoured that [he] had a chance to deliver that line."
If anyone hasn't got Police Squad on DVD then you're missing out, they were genius, and his delivery overall is even more deadpan.
An amusing interview with the man..... http://www.empireonline.com/interviews/interview.asp?IID=1127&WT.dcsvid=SilverpopMailing&WT.mc_id=120310%20newsletter%20(1)