How can I motivate my fellow players?

You need to determine what kind of players you are playing with. Some enjoy story lines, pursue growing their characters, most just like to kill stuff. Its' best to play with like-minded players.

Always have game chat before the game starts. The best thing to do is chat about what you would like to see from the game with each other while the GM is setting up. I'm sure the GM will be eve-dropping, and some of it may be incorporated into the campaign somehow. It's just always good to get everyone psyched about playing before you play.

Take Breaks! about every 30 minutes of storyline, or after a great battle, everyone should take 5 or 10 minutes break. Step aside and reflect/discuss what just happen, what was fun about it, etc...

I've encountered the same dilemma. @Sardathrion's point about honest communication is right on the money. Discussing what's working and what's not working will solve most problems. That said, sometimes it's difficult for players to pinpoint what they are missing, or why they are distracted. You may want to suggest to the GM that opening each session with a bang might help get everyone engaged.

The GM could have an NPC come to the PCs with a problem that requires investigation. A thief could steal something important from one of the PCs. The PCs could get hauled before the magistrate. Whatever the event, it needs to be dramatic, and the outcome must be meaningful; if the PCs don't handle it well, they'll pay some sort of price. This will get the players emotionally engaged quickly.

Once the players are emotionally engaged, it may be easier to keep the flow of play moving smoothly.

In teaching, we are taught that there are 6 C's to motivation. These are: choice, challenge, control, collaboration, constructing meaning, and consequences. I would suggest that to motivate players in ways other than those already mentioned (recaps, social times, frequent breaks, etc) I would try to focus on helping the DM/GM provide more of the 6 C's.

With regards to choice, try to think of what gaming choices you can promote/support that would favour the interests of the unmotivated players. For example, if they are bored by combat, start championing group approaches that favour diplomacy, evasion, or intrigue.

When thinking challenge, try to pick fights/missions that suit but stretch the abilities of the members of your group. Fight each other if you have no appropriate alternative, but pick and choose your battles so that everyone can contribute with a real sense of (manageable) threat present.

Giving players control over how and when they will participate promotes player buy-in, increasing motivation. Try not to let any one voice or "group think" dominate your play. Be the "devil's advocate" when you can.

Collaboration is implied in any gaming group, but you can take it on as a personal responsibility to augment the effect of another player's desired course of action by aiding them in ways that give bonuses or further alternatives for action - even if it's not something you might normally choose to do.

Constructing meaning is harder for a player to accomplish than a DM/GM, but basically, in order to motivate someone to do something, it needs to personally have meaning for that player. Perhaps, in character, you can start wagering gold on the outcome of various encounters, or suggest why what the group is doing next should matter to the unmotivated player (in character).

Finally, consequence is a matter of recognizing achievement when it happens. Showcasing an unmotivated player's achievements motivates that player to continue along that line in the future. Be descriptive where possible.

Likely the issue is one of the following:

Why are we here?

As has been addressed in other threads both here and across the internet, we game for different reasons. Some people like to engage their inner-thespian, while others have a very stressful job and only want to picture those annoying people when they "HULK SMASH!!!" Still others are there because they like the people and will play D&D with the group just as willingly as they will play Bridge or watch cult-classic movies.

Why is everything always about YOU?

Since you mention that you are 100% engaged, but the other players are getting increasingly "meh", I would suspect that the plot is very closely focused on your character just now. Maybe the DM wrote a campaign that would be fun to play and you happened to create the perfect centerpiece character for that adventure, maybe the DM has been increasingly writing adventures around your character's wishes/desires. While the centerpiece character is having the time of their life, the others are getting more and more bored about stuff their characters just don't care about. I once played a game where we discovered a continent. My character was the Royal Cartographer and was in heaven mapping as much as the DM would let me map, whereas all of the other characters were mostly urban-centric characters on a great wild continent with no cities, and no real way to make money or do what they imagined doing during character creation.

How do we get out of this hole?

Whether it's a hole of plot-lines circling a character or the tone of an adventure matches up with one player at the exclusion of the others, the solution is the same. Sharing.

If it's a tone issue, bring in the elements/motivators for the other players. If you like long dialogs but the other players don't, either handle the long chats off-line via email, or decide that you don't need to have the entire life story of every minor NPC. If you want non-stop action and the other players enjoy talky scenes, stop running ahead and pushing the plot forward and engage in some dialog. If you like combat but the others want to explore, choose to not do the frontal assault for once ("Normally I would suggest we bash down the door and do the direct thing, but I've got a bad feeling about this place..."). Variety in your actions not only keeps things fresh but lets folks with other interests put forward their ideas.

If the issue is that you are the one getting all the "plot cookies", figure out what each character can do for YOUR character and take advantage of their skills/abilities in your schemes to pull them back into the game from there. In my Cartographer adventure, he would routinely spend days at a time away from the main camp, the bard was an excellent messenger who could relay the news of the camp (about 50 NPCs) and carry messages between my character and the civil authority. The Wizard was able to pick up cartography pretty quickly and help my character out with rough sketches to make the map more accurate and loved the opportunity to find new magical reagents and sources of material components for spells. The more I was able to bring in the motivations of the characters and the motivations of the players, the more the group was pulled back together.

Regardless of which is the problem, I would also drop an email to the DM and bring your concern(s) up. The DM can much easier alter tone/plot to work in players who are bored, but as someone has been an overworked DM it is sometimes hard to see that 1 person is lapping up the plot and the other party members are getting bored. Also, sometimes the "bored" player is not bored, but distracted. There have been a few times where personal issues have kept me from really immersing myself into a game.

Props and Cues: To echo what Sardathrion said, gaming props (like the single article of clothing the Penny Arcade Guys wore) can pull players into character much like how costumes help actors assume a role. Music can also be a big way to draw people in. This can be really overdone to the point where the GM spends as much time crafting the perfect playlist and running the music as he spends prepping the session and running the game. Kept to proportion though starting music (especially if its a theme song) and some appropriate music to play in the background during the dice heavy combat sessions can rouse players and draw them in as well.

Set social time: For some groups, D&D is their main time spent with each other and everyone likes to catchup on life, talk about the latest episode of insert fantasy/scifi series here and in general enjoy each others company. This is not a bad thing, its probably why you started playing together or came about from playing together. What is bad is if it becomes muddled with actual game time. Setting aside time (also nod to Sardathrion) before the session begins with an agreement from everyone about how long it is acknowledges this and helps you to get it out of the way before gameplay commences.

"Last time on Dragon Ball Z..." Quick, witty recaps (perhaps purposefully cheesy) can refresh player's memories and put them in the right frame of mind quickly. Unlike the actual recaps from Dragon Ball Z, keep these ones to about a minute long.

Have set break times perhaps people have actual physical or mental fatigue mid-session. Having a designated 15 minute break every 2 hours you play can let people stretch, take a breather, and maybe grab some snacks without interrupting gameplay and feeling stressed out about it.

Communication is Key

As I am the DM I usually worry about motivating my group from my side of the DM screen and as a DM it can be very disheartening to see you players lose interest but your question entails quite a different dilemma but in my opinion is a very interesting and important one. As always communication seems to be the key.

I would ask them what's going on,why they don't seem to be enjoying the game. Maybe you should point out what you see going on to your GM and ask him to get everyone together and talk about what they like, what they don't like and also talk about everyone's expectations.

If your GM doesn't have the time you could contact your fellow players get the needed information and get they're opinions to the GM.

This may be a situation where you have to take charge instead of relying on the GM which seems to be exactly what you are doing.

Meta: Do the grown up thing: Ask them up front that you perceive them losing their enthusiasm for the game and that you would like to know if it's true (or just your perception) and if so, why this is happening? Then, you can resolve things so that everyone is happy.

In game: If the problem is one of distraction, then set aside time to drink coffee and chat before the game starts. Set a timer and once it gets going: just stay in character at all times!. Even when you go to the toilets, or answer your phone, or order pizza: just be in character. Costumes props can help as well as long as they are not too disruptive in and of themselves. Suggest the use of mood lighting and mood music as well. Removing sources of distractions can help a lot: no more laptop (with frikking solitaire), comic books, or film magazines...

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From a comment: how do you get the other players to agree to do that? By that, I assume the second point. If you cannot get the other players to listen to you, then find better friends. Otherwise, you engage the grown ups in the room. If you cannot reach a common ground, find other players. Or just try it, they may pick on that and go with it. You could learn manipulation, blackmail, and confidence trickery so that you can force the other players to do your bidding. However, in my not so humble opinion, that makes you a scum bag. Mind controlling your fellow players is in the realm of bad super heroes plots. <pinch of salt added>. ^_~

If you have personal plots, try to involve characters on them. Not as a whole, but individually. If a PC has criminal contacts, try to use them in your plans. If one is good at forgery, involve him in the creation of false documents. Seek their help and make them feel special.

This is what I do when I see a PC is getting outside the story.