The most crucial role of a Game Master is that of the storyteller. Yes, you need to keep track of gazillions of rules, and game metrics, and rolls, and statistics, etc. etc. If the story doesn't flow, the players will get bored; the discussion will turn to things happening in the real world. Before you know it, that immersive experience you spent weeks developing and writing will fade away, leaving nothing but almost empty pizza boxes and soda (or beer) cans.
The biggest threat to the immersive experience is combat. Yup, I know, that's supposed to be the exciting part. But I see many GMs, especially new ones who get bogged down in the numbers and forget to keep the story going. Remember, the thing that makes a great story is not the plot; it's the details.
Let me give you an example, Sports Broadcasters. Can you imagine watching golf or baseball with the TV volume turned off? I'd be asleep on the couch within 15 minutes. Now imagine listening to a baseball game on the radio, that thing we all used to have in our cars before smartphones and podcasts. You can't see the game, but a good sports broadcaster can make you feel like you are there. Sound familiar. You aren't fighting a pack of Gnolls, but a well-narrated combat encounter makes you feel like you are there.
You see, Sports broadcasters like GM's do more than give the play-by-play;
"He hit the ball over the fence."
"Morgana rolled a 19 and hits the Gnoll for six damage."
Sports broadcaster and GM's provide a live running commentary to capture the game's feeling for viewers and listeners. Compare the previous descriptions to color commentary.
"It might be, it could be, it is... a home run" (Harry Cary, Wrigley Field).
Now an example in a running D&D combat narrative.
"A Nineteen? The Gnoll had thrust at you with his spear, but you danced aside. Using your momentum, you fuel a backhand slash of your short sword, damage? Six. The blow cuts deeply into the Gnoll's bicep. It yelps in pain. You have its attention."
Do you see the difference?
Right now, you are saying to yourself, "that's easy for you to suggest sitting at your desk writing a blog entry. You don't have to keep track of initiative, damage, modifiers, whose turn is it…" Your right, and for the most part, neither should you. Once the initiative rolls are complete and jotted down on a piece of paper, you can focus on the narrative. It's not the good ole days of AD&D when you had to roll initiative for every round and worry about segments and a score of other modifiers to your order. I admit I still like the concept of everyone declaring what they are going to do up front before the first attack die hits the table. Remember you are about to play out, which in theory are simultaneous actions, all taking place over a period of 6 seconds. Your ability to maintain the narrative depends on "training" your players to help out.
The in-game time may be six seconds, but lack of preparedness on the part of your players could make that 60 minutes (only slightly exaggerated). Take some time before your game to let the players know what their responsibilities are during combat. After that initiative roll, give them a minute to process the situation and decide on their response but only a minute. If your group is new to the game, let them know that their turn is not the time to franticly flip through the PHB to see the damage and area of effect on that fireball spell. Remind them that they are fighting as a team, so looking at that underused second-level Enhance Ability Spell may be a good idea.
When you get to each player, they should already know what they will do, roll the dice, and let you know the result. If they were prepared, that result could be filled with color.
Player: "Yeah, 17!"
DM/GM: "That hits!"
Player: "I swing my war hammer right at the dagger Gordo (the Rogue) sunk into its shoulder" (rolls damage) "Four Damage."
DM/GM: You hit right below the dagger spinning it around. The Gnoll turns back to you, teeth bared. It's hurt, just enough to make it really mad.
After that dialog, you jump right to the next player or monster in the initiative order. If that player isn't ready to push them, add some pressure. It's combat, after all! You don't have to be mean, but you can add to the tension and the stress. Delivered correctly, your prodding will enhance the whole experience. But that's another blog topic.