The following example assumes that you are starting out with a Sub Station Alpha script and you want to use it to display subtitles with mplayer. Most of the following applies to other script formats as well, but there will be minor details which differ.
Please take note of the following limitations in the mplayer subtitling engine, since they will make your life more difficult than that of a traditional subtitlier:
mplayer cannot simultaneously display two different subtitles on the screen at the same time! mplayer does not (yet) allow you to place a particular subtitle in a special position on the screen, different from all of your other subtitles! mplayer cannot automatically generate "balanced" subtitles; that is, multiple line subtitles where the different lines of text are as close as possible to being equal length. If you prefer balanced subtitles then you will have to manually add line breaks into your script. mplayer does not understand most Sub Station Alpha formatting codes, so any such codes in your script will most likely result in mplayer displaying junk on the screen.