“Being solemn has almost nothing to do with being serious, but on the other hand, you can't go on being adolescent forever, unless you are in the performing arts, and anyhow most people can't tell the difference. In fact, though Americans talk a great deal about the virtue of being serious, they generally prefer people who are solemn over people who are serious.
In politics, the rare candidate who is serious, like Adlai Stevenson, is easily overwhelmed by one who is solemn, like General Eisenhower. This is probably because it is hard for most people to recognize seriousness, which is rare, especially in politics, but comfortable to endorse solemnity, which is as commonplace as jogging.
Jogging is solemn. Poker is serious. Once you grasp that distinction, you are on your way to enlightenment.”

"Why Being Serious Is Hard" (p.17)
So This Is Depravity (1980)