Luke 24:17

‘And he said to them, “What are these things that you are talking to each other about with one with another, as you walk?” And they stood still, looking sad.'

The ‘Stranger' then asked them what they had just been talking about. It suggested that He had been observing them for some time (as he might have done if He was slowly catching up with them). At these words they stopped, the grief apparent on their faces. We have here an indication that the account was told by someone who was there. His words had brought them to a halt, and they remembered it well.

Now it is true that a consummate storyteller might have introduced such factuality into a fictional account, but we know from the crucifixion narratives that Luke was far from seeking to do things like that. He was telling things as they were without embellishment. Thus there is no reason for thinking that it was any different here.