For writing about an actor on a televison show (Keifer Sutherland on "24") use the word "on" before the televison show title.

Why "in" for film and "on" for Televison show?

You see an actor "in" a film, by watching a projected image on a screen, for a actor on a televison show, you see the actor "on" a television screen(particularly your own).

For the advanced: both a film and a television show are performances but to be in a film is important as being on a television show.

which sentence sounds right to you:

Previously in "24" or Previously on "24", even try this "I saw Sean Penn in "Mystic River"" or "I saw Sean penn on "Mystic River.""

the program in question "Mystic River" is a Motion Picture, so sentence one "Penn in "Mystic River" is correct.

If "Mystic River" was a television series sentence two would be correct.

for the Line in the Television Series "24" Line two "Previously on "24"" is correct.

"24" is a television show.