Radio shows and podcasts alike are fairly topical, and podcasts are especially so since they tend to be very focused on something specific and are not as broad as radio. When you are copywriting podcasts for a specific day you should draw out a few stories, ideas, or topics that are specific to your individual show so that you have a thread to draw it through.
The nature of this is going to be very dependent on the format that your podcast takes in general. For example, if this is the podcast of a filmmaker or musician then it is likely going to be something that happened in their life or a news story that is relevant to their perspective and work. If it is a podcast on comic books then it may be on a specific artist, character, event, or storyline. This does not need to be the whole of the discussion, but should be something that is introduced in the opening monologue and then returned to throughout the podcast production.
If you have a guest, it is best if they are around the topic, but since these are often chosen shortly before the recording and when you write the podcast script, you may have to improvise. Either way, if you have a guest it is important to continue the commitment to the topic by discussing it with them. You should not, however, remain focused on the topic if the guest is not contributing. This can always be edited out in the audio mixing program that you are using, such as Pro Tools.