When dealing with monsters who have resistance or vulnerability to a specific damage type, I've been having my players roll the expected damage and I'll half or double it behind the screen. I've recently found a DM who tells the players outright if the monster is resistant or vulnerable, and, given the example of vulnerability, has them roll double dice instead of doubling the damage.
Should the player roll damage and the DM secretly halve or double it depending on the situation of resistance/vulnerability? Or should the DM tell the player that the monster has resistance/vulnerability and have the player roll half/double the damage dice?
Player Bob is playing Bob the Barbarian. He hits a (DM-controlled) Grick with his club. The Grick is resistant to non-magical bludgeoning damage, and the weapon is non-magical.
Do I, as the DM, tell Bob the Barbarian before the damage is done that the Grick is resistant to non-magical bludgeoning damage and have him roll and halve the damage himself? Or do I hear his total damage roll, halve it behind the screen and explain that while the hit was good, the Grick doesn't appear to be as fazed by it as he's used to seeing monsters when he hits them this way?
(Does the player roll it [knowing ahead of the damage roll] or does the DM take it in and explain it through the story?)