With the scene footage in front of you, you can play your piece and see how it moves the

scene forward and how it works with the editing. If you don’t have editing software, just play the song in iTunes and then play the video. It won’t exactly match but you’ll get the idea.
Remember: A lot can be fixed by the music editor in terms of matching downbeats and lyric phrases to dialog and action.

With just a description of the scene, you have to work blind. Try to go to the underlying dramatic or comedic purpose. What will support the emotion of the scene?
90% of the time a written description will be all you have to go on. Sometimes try to include a “wild pitch” — maybe not what they’re asking for — but you think it could work.