Well, in the Gygaxian playstyle there isn't a distinction because there's no such thing as PC motivation. The players want to win by beating the dungeon and they use their PCs as tools to achieve that aim.
In modern play, the players engage with the GM's content because they've turned up to play a roleplaying game, they want the session to be entertaining, and they don't want to be impolite to the GM. Those motivations seem quite different from PC motivations, though maybe there's a parallel with the 'thrillseeker' motivation I mentioned in the OP.
Well, in the Gygaxian playstyle there isn't a distinction because there's no such thing as PC motivation. The players want to win by beating the dungeon and they use their PCs as tools to achieve that aim.
In modern play, the players engage with the GM's content because they've turned up to play a roleplaying game, they want the session to be entertaining, and they don't want to be impolite to the GM. Those motivations seem quite different from PC motivations, though maybe there's a parallel with the 'thrillseeker' motivation I mentioned in the OP.
Well, I think that's only true to the extent that it was the early stages of RPGs and the game was built around the motivations the players had for their characters in Gygax's and Arneson's campaigns. The game was new, it had a level of re-playability that I don't think any other game had at the time, and the wonder and excitement came from working through the latest creative ideas of the DM, so PC motivation was only needed to extent that it got them going in the adventure.
I do think that the strengths and focus of the people involved are important, though. Gygax was clearly interested in the game/adventure design part of things. I think this was both a natural inclination and also grew, or was reinforced, by being the primary DM/designer at TSR at the time. In other words, it wasn't his job to focus on PC motivations, that was up to the players to address. But I think the rules also reflect the PC motivations of his players at the time. It wasn't long before articles from folks like Ed Greenwood started exploring the other side of role playing, with lots of lore and complex motivations of NPCs and, by extension, PCs. But then he came from a different perspective. He thought the game was a great opportunity to bring to life the world and characters he'd been creating in his fiction. His approach has always been a view of the world from the character's perspective. Whether that character is the villain or a PC.
I'm not sure, but I think Dragonlance was the first Adventure Path style approach where there is a relatively linear plot on top of the adventure points. These three approaches highlight three major approaches to PC motivation.
Gygax leaned toward a sandbox, with PC and NPC motivations only really required to the extent that they intersect with the action in the adventure. So for an NPC you really needed to know their present disposition, and a basic understanding of who they are, which usually amounted to their profession. You don't need to know any more about the merchant, just address the reaction rules while you haggle. The extent of PC motivations, however, is entirely up to the players. So yes, many people reduce that down to the bare minimum of character motivation, and it often matches player motivation. But it doesn't preclude PC motivations, nor prevent them from being different than player motivations.
Greenwood was seemingly the opposite - it was all about the motivations and personalities of the PCs and NPCs. Cities aren't just home bases to equip and recuperate, they are often the center of the adventure. And the adventure may have nothing to do with monsters and dungeons. It could be a mystery at a party in a noble's villa. White Wolf and other storyteller games would expand greatly upon this approach. The world is often richly developed, as are the NPCs, politics and such. All to provide more potential hooks for the PCs to latch onto based on their motivations.
Dragonlance was something different. It took the linear design of a dungeon and applied it to a campaign in the form of a plot. Where Gygax might have the barest hint of a plot, and Greenwood has lots of ongoing plots and schemes behind the scenes, the Dragonlance saga was all about one central plot. The PCs are roles to play in much the same way an actor adopts a role on stage or in a movie. As the player you have a fair amount of freedom in your interpretation, performance and actions, but little impact on the larger, overarching story. PC motivations are quite important, but are often provided to you by the author.
What's kind of interesting is how these different approaches have been mixed up in different ways. They aren't mutually exclusive, and the focus of each of the principles was different. Gygax was focused on the dungeon design. The design of the adventure itself was a driving factor or a focus. Part of being a skillful DM was the clever design of your dungeon and adventure.
For Greenwood, it is still a sandbox that allows exploration of the setting, the focus is as a sandbox that explores the characters. It's not that far removed from a Gygax sandbox in that the setting is usually wide open for exploration. The real difference, I think, is that the narrative is more driven by the players/characters than the DM, and the focus is on the narrative and not the dungeon. The DM can still have a lot of input in the narrative through events and NPCs, but it's more suggestive, in the form of hooks, and if the PCs take the hook, then the adventure heads in that direction. It's more of an evolution of the Gygax sandbox to me. The design intent may be less about being clever and more about being immersive.
Dragonlance, on the other hand, was neither. It was playing a part in a story. The setting wasn't really a sandbox (if you were playing the adventures) and neither were the characters. Instead it was an exploration of specific characters in a specific set of circumstances. Where both Gygax and Greenwood were conducive to the players having many PCs, and character death was quite possible, Dragonlance was more about exploring a single character in more depth. You could, of course, in either a Gygax or Greenwood campaign explore a character in a similar fashion, But in Dragonlance, that was where you could explore the farthest as other avenues were more restricted. In theory this is also about being immersive, but that's achieved only to the extent that the players can adopt the roles of the PCs, combined with how skillful the DM is in making the adventure feel like the players have control of the plot.
As RPGs as a hobby have matured, exploring a single dungeon is not enough, and players expect there to be a setting holding things together. Modern players expect a relatively detailed setting, the ability to explore their characters (often mechanically instead of motivationally/personality wise), and a story to grab their interest.
Adventure Paths make character motivation easier but more restrictive. If you create the character for the AP, then the motivation is addressed at character creation and typically won't be a necessary factor to keep them within the AP. They have the same sort of plot structure as Dragonlance, but without requiring specific characters. The final outcome of the AP usually only has a few potential endings, but the characters have a decent amount of freedom on how they get there. Like the Gygax approach, character motivation is largely up to the PC, with the caveat that it's best to ensure that the motivation allows the continuation of the party/AP.
Story Now games are centered around the idea of character motivation as the driving focus of the entire game. That the action and events should center as much as possible around the characters and their motivations. Dungeon design, setting design, and such are often of minimal importance, and sometimes shared to various degrees with the players. In many cases, a prepared setting or adventure by the GM is discouraged, with the adventure initiated and developed through play in relation to the PC motivations. I think this is sometimes about as far from a Gygax sandbox as you can get.
While I don't particularly care for the Story Now games I've tried or checked out from a design/mechanic standpoint, what they get right is that the DM should be engaging with the motivations of the characters in the course of the campaign. There are many ways to do this, but as long as that's happening, then the players aren't just "engage(ing) with the GM's content because they've turned up to play a roleplaying game, they want the session to be entertaining, and they don't want to be impolite to the GM."
I'm not sure I'd consider that there's a group that qualifies as "in modern play," but certainly in modern game design (including 5e D&D, although not necessarily promoted within the APs), the intent is to engage with the GM's (or author's) content
as the character and are as immersed in the experience as the GM. You're not really coming to be polite to the GM and experience their game. It's about a collaborative and shared experience, with each game defining the roles and boundaries between player and GM differently.
D&D in particular isn't great about promoting characters with complex motivations, but it most definitely supports them.
