He was well known as a dramatic actor before pivoting into comedy. That's why he got the role in Airplane, they wanted known dramatic actors to play all that absurdity straight. I was just re-reading about it to avoid saying anything dumb in this comment, and learned that when asked about being cast "against type" in comedies, he said that he'd always really been cast against type in his earlier dramatic roles, and comedy was what he wanted to be doing from the start. Glad he got his shot!