When a movie or a book title is in a sentence when is it underlined?

Movies titles (and TV show titles) are supposed to be italicized. Short movies, or episodes of TV shows, are put into quotations, for example, episode, "Squeeze" of The X-files.

Similarly, titles of novels are italicized, and titles of short stories are put in quotations.

Underlining movie titles is somewhat archaic. I think that underlining was used when people wrote with typewriters, instead of Word processors on computers. Typewriters can't italicize letters. When the book was sent off to the printers to be published, underlining was a cue to the printer that that portion of the text should be printed in italics.

At least, that's the explanation I remember reading in Stephen King's Misery. But unfortunately, I can't find that passage now, so I can't check and say for sure.