64 It may also be asked how it is that Mark says: “And they went out quickly, and fled from the sepulchre; for they trembled and were amazed: neither said they anything to any man; for they were afraid;” whereas Matthew's statement is in these terms: And they departed quickly from the sepulchre with fear and great joy, and did run to bring His disciples word. The explanation, however, may be that the women did not venture to tell either of the angels themselves—that is, they had not courage enough to say anything in reply to what they had heard from the angels. Or, indeed, it may be that they were not bold enough to speak to the guards whom they saw lying there; for the joy which Matthew mentions is not inconsistent with the fear of which Mark takes notice. Indeed, we ought to have supposed that both feelings had possession of their minds, even although Matthew himself had said nothing about the fear. But now, when this evangelist also particularizes it, saying, “They departed quickly from the sepulchre with fear and great joy,” he allows nothing to remain which can occasion any question of difficulty on this subject.
Source: The Harmony of the Gospels (New Advent)