Beyond simple surprise that Jesus was alive again (hardly an expected outcome — unless they had been listening), it seems that Jesus prevented them from recognising him:
When he was at table with them, he took the bread and blessed and broke it and gave it to them. And their eyes were opened, and they recognized him. And he vanished from their sight. They said to each other, “Did not our hearts burn within us while he talked to us on the road, while he opened to us the Scriptures?” (Luke 24:30-32)
Their eyes were opened — it strongly implies an active agent, since it happened to both at once.
It’s also possible that Jesus’ appearance had changed. After 3 1/2 years living on the road, carrying the world on his shoulders, he was probably a bit wiry and weather-beaten. Perhaps he was healthier afterwards? Obviously it’s easy to speculate that perhaps his appearance was even more altered by immortality, but even so, he was still recognizably Jesus once their eyes were opened.