Theophylact of Ohrid

If Matthew says that the Angel was sitting on the stone, while Mark says that the women, having entered the tomb, saw him sitting inside, one should not be troubled by this. For they could have seen the Angel sitting outside on the stone, as stated in Matthew, and could have seen him again inside the tomb, as having preceded them and gone in. However, some say that the women mentioned in Matthew were different from those in Mark, and that the Magdalene was the companion of all, as the most zealous and fervent.

Commentary on Mark ↗

Augustine of Hippo

Either let us suppose that Matthew was silent about that Angel, whom they saw on entering, whilst Mark said nothing of him, whom they saw outside sitting on the stone, so that they saw two and heard severally from two, the things which the Angels said concerning Jesus; or we must understand by entering into the sepulchre, their coming within some inclosure, by which it is probable that the place was surrounded a little space before the stone, by the cutting out of which the burial place had been made, so that …

Catena Aurea by Aquinas ↗

Gregory the Dialogist

But the women who came with spices see the Angels; because those minds who come to the Lord with their virtues, through holy desires, see the heavenly citizens. Wherefore it goes on: And entering into the sepulchre, they saw a young man sitting on the right side, clothed in a long white garment; and they were affrighted. (ubi sup.) But what is meant by the left hand, but this present life, and what by the right, but everlasting life? …

Catena Aurea by Aquinas ↗

Bede

And entering into the tomb, they saw a young man sitting on the right side, clothed in a white robe, and they were amazed. Entering from the east into that round house carved in the rock, they saw an angel sitting on the southern part of that place, where the body of Jesus had been laid, for this was indeed on the right side. For surely the body, lying supine with its head towards the west, must have its right side towards the south. …

On the Gospel of Mark ↗