Radio

You want your listener to immediately get what's going on, and your first five seconds is the place to make sure this happens.
Overwriting is the most common mistake people make in radio, especially juniors. Be a genius. Underwrite.
Don't waste time explaining things
Screenwriter William Goldman advised, “Cut into a scene as late as you possibly can.” Good advice. Crisp self-editing like this keeps your story moving along with a minimum of moving parts.
Keep an ear cocked for those sentences where the actors stumble. If more than one actor has a problem with a line, it's likely the line that's the problem, not the actors.
Have your voice-over talk like you talk. I find a flat read is almost always best.