When a journalist calls… The first and most important thing you need to do is ensure you understand what the journalist is looking for. As well as understanding the topic of the interview, understand the deadline they are working to, and the requirements for their medium.
For example, the journalist may be looking for someone to provide an urgent, recorded “grab” for the next hour’s radio news that only requires you to deliver two or three sentences of information. Or, they may be on a weekly deadline and need to arrange a lengthier interview for a later time or date.
In the case of radio interviews, some will be pre-recorded ahead of time and in other cases they will be performed live. Make sure you understand what kind of interview you are doing, and what will be required. In cases where an immediate response is not required, it is usually fine to tell the journalist that you are happy to help, but need to take some time to gather your thoughts before calling them back at an agreed time.