Exactly, sometimes you have someone who is clearly in pain and/or close to the edge of consciousness and you know that the second you stop talking they are just going to drift off which is not what you want ... but you can't start having a deep conversation because you're also assessing the situation - what supplies do I need, what supplies do I have, who is around to help, what do I need them to do, who else needs help, has anyone called 999, does anyone know the information the 999 operator requires so I can hand the phone to them and concentrate on keeping the casualty as comfortable as possible etcetc - as well as trying to keep everyone else calm and keep control of what is happening, all in the span of a couple of minutes.
During all of this you need to keep up a stream of information between you and the casualty - initially you can focus on the facts "Can you tell me your name? Can you tell me what happened? Has anyone called an ambulance yet? Can you tell me where it hurts etc" but then you get to the point where you have dealt with the immediate danger, you have all of the facts but you are still waiting for the ambulance to arrive. In those cases you need to keep the casualty talking and responding but the problem is this is a relative stranger and you have no idea what things they're interested in or what their life experience is etc so it's easy to just make light hearted conversation based on what is happening at the moment.
A couple of weeks ago I was called at work to an elderly gentleman who had fallen outside our store, he was a bloody mess when I arrived and his wife was hysterical ... I managed to talk to the gentleman and found out he was on blood thinners so whilst it looked awful he actually wasn't as badly hurt as it first appeared but still needed an ambulance as he was feeling a bit woozy due to the blood loss and needed to be stitched up in hospital and checked over but we weren't top priority for the ambulance list. Because of where he had fallen and the size of his injury it was too dangerous to try and move him far so I managed to get him sitting up and then we had screens put up to shield us from anyone gawking while we sat and waited for the ambulance.
By this point he was massively embarrassed at the whole situation and kept trying to say he was fine and just needed to go home but by making little light hearted comments I got him chatting, and the comment "You're going to have some impressive looking bruises, I hope you've started practicing your story about how the other guy turned out" etc got him talking about the fact he was a champion boxer when he was in the army back in the day and then lead on to a natural conversation which filled the time until the ambulance arrived, let me keep an eye on how alert he was (whenever he started trailing off I could ask him questions about what he'd just said to keep him going), took his mind off the injury and also calmed his wife as we could include her in the conversation... All of which was achieved from a jokey opening comment.