I have a grade school memory – I was probably 12 – of a dungeon master penalizing a set of player characters for acts of hubris. On the surface this may not seem so terrible. However, the DM and player weren’t actually playing D&D at the time. Rather, the player was talking about what his characters were up to. I still remember his argument: not only does a DM have absolute power over his game, any DM has complete power over any PC or set of PCs. I wasn’t directly involved in this campaign, but it seemed clear even to me that a DM has no extra authority when the game is not being played. Hopefully this point is not controversial among enlightened gamers such as us.
This points at a more fundamental point that applies beyond the playground. The power of the DM is exactly equal to the power given by the players. It is often assumed that the DM has absolute control over the campaign, after all the word ‘master’ is there in the name. While Dungeons and Dragons certainly works best when the DM is granted autonomy and decision making control, the details should be made explicit.
In the game I play in, our DM was once accused of ‘cheating’ when he interpreted a rule in a monster’s favor, contrary to a precedent published by Wizards of the Coast. We were winning the battle handily, this was a major boss, and he was adding some challenge in order to enhance the fun.
The majority at the table supported his decision. However, we had never explicitly talked about what we thought of a DMs power to adjust things on the fly.
That second example should illustrate the other end of the spectrum. Considering that instructions to create monsters are in the DM’s guide, it can be assumed that he can make a tweak here and there to make a game more exciting.
Not everyone will answer power dynamics questions the same way. This especially applies to questions related to house rules and PC options. While there’s no magic answer, clarity up front is always the best policy. As a player, ask the group what they (including the DM) think. As a DM, do the same, preferably before the heated moments come.