Commentaries:
"Fool" means someone who is inconsiderate, somebody who is not considering the knowledge, the truth, the teaching, that they already had. They were not considering the events of the day in the right context. They were inconsiderate. He says it as if He expects them to come to the right conclusion about who was walking beside them and expounding the Scripture.
He also calls them "slow of heart," which means someone who is backward or not easily persuaded. A child-like mind was not yet developed in these men. So what do they do? They easily give in to doubt. This continued to the point that He actually had to make Himself known to them miraculously.
John W. Ritenbaugh
We Are Unique!
Other Forerunner Commentary entries containing Luke 24:25:
Matthew 9:29
Luke 12:54