A favorite passage for many reasons. And so very easy to preach. But sometimes the easy passages to preach become difficult as you have preached them so many times. Perhaps this will give you a few insights.
1. They did not recognize him. Why not? Did he look different? Did he sound different? Or was it simply because, well, he died! Who expects the person they saw die 3 days ago to be walking next to them?
2. The Scriptures revealed to them who he was. The Gospels are careful to point out that Jesus as Messiah was a fulfillment of the prophecies.
3. They recognized him at the Table. Note he didn't ask if they believed before they sat down or even before he broke the bread. My tradition, Wesleyan, believes in an open table. We do not believe that there is any requirement for sharing the Lord's Supper except for a desire to know Jesus.
4. This passage may be used as a lens in which to interpret all of Scripture.
a. We come with limited knowledge
b. The spirit of Christ reveals the Scriptures to us.
c. The Spirit reveals His Word at the Table.