We have a party of several players and there are some things that are not clear from the rules. Can you guys help?
Noticing Theft From NPCs
Do other party members always know if one of them steals an object from an NPC? Or does he has to announce it. The reason for asking is that rogue constantly steals stuff from NPC and keeps to himself not telling other.
You can use the same rules for that as you would use if NPCs (like guards) were trying to notice him stealing. He uses Sleight of Hand (and possibly Stealth), and they use Perception. If they don't know he's stealing but are in the area and entitled to a check, roll it in secret and only tell them if they would notice something.
By the rules, a player trying to be sneaky opposed to other players is no different than one trying to be sneaky opposed to NPCs. At the table, it requires some different considerations (the player doing the stealing probably doesn't want to announce it to everyone else at the table).
Stealing From Players - By The Rules
Can rogue steal from other party members?
Yes.
Once again, use the same rules as when stealing from NPCs. Typically you'd also make these rolls in secret, so the target player doesn't put together that he has to roll at the same time as another player and poof, stuff goes missing.
If you need to talk to a player in secret, this question has some good answers on the subject.
Stealing From Players - By Social Convention
This is a whole other story. Some players are totally okay with being attacked/robbed by other players. Some players really hate intra party conflict, and want to play in a party that's all one happy team.
It's probably a good idea to talk to your players and figure this out before it becomes a thing. Having a player get robbed who really doesn't want that in the game will lead to problems with the players not getting along.
Conflicts
How do you handle conflicts within the party? Can they use intimidation on each other? And they have different alignments.
You can use Intimidate on PC characters. Generally speaking, I don't allow PCs to use social skills (Intimidate, Diplomacy, Bluff, etc) on other PCs, but that's because I've found that my players prefer to deal with each other directly in those situations and not let the dice dictate things. That's not a rule, that's just how the players at my table prefer to handle it.
If one of your characters is very much "punish criminals and traitors" type, it may come to blows pretty easily if the thief is outed.
If conflict happens between the characters (or players), there are a lot of questions here on RPG.SE with advice on how to deal with it.
Here's one. Here's an article from somewhere else on the same subject.
Your Table's Social Contract Is The Key
I strongly recommend bringing all of these points up with your own group of players, out of character, to discuss how they feel them about them. Do your players think they should be able to do things without the other party members knowing? Do they want nothing kept secret from all players, feeling comfortable without play without letting their characters act on knowledge they shouldn't have? Or would they prefer to be kept in the dark of any knowledge their own characters wouldn't know?
And how do they feel about tricks and skills being used against one another? Do your players, again out-of-character, feel comfortable with letting the rogue PC steal from them? Do they want to role-play that out, struggle with those kinds of conflicts in-character? Or would they very much rather not bother and get on the business of adventure and do-gooding? If it's the latter, tell your party rogue in no uncertain terms he's not allowed to steal from the party. Appease the little klepto by giving him plenty of situations to pilfer from the party's mutual enemies, and everyone will be all the happier.
If you're asking for advice on which way to lean, no one can do anything more but share how they handle these situations at their own tables. In my case...
Secret Knowledge is A-OK
I love suspense, and in particular I love to create suspense through dramatic irony, which is when the audience knows something the characters don't. When the audience and characters are one-in-the-same, I create dramatic irony by only revealing crucial information to certain players (and thus, certain characters). What those characters choose to do with that information, and what they choose to share, is ultimately up to them. I once had a character who had detect evil at will, but was generally a moral relativist. I'd reveal that information to him every time a new NPC was encountered, but he never shared it, because he never felt the information to be particularly relevant. This has always worked great for my table.
Game Mechanics are PC vs NPC ONLY
This is a hard and fast rule of my own. I feel very strongly about it, but I also recognize that other players feel very differently. At games I DM, the game mechanics exist to resolve conflicts between the PCs and my NPCs only. PCs do not roll dice to oppose one another. Bluff, Sense Motive, Intimidate... these are skills I reserve from PC/NPC interactions, not PC/PC interactions. I don't even allow attack rolls (including grapples) any more except in extreme circumstances that I've planned for and telegraphed in advance (the party tracking down and facing a powerful Illithid Dominator, for instance). The only real exception I make to this rule are Perception checks; party members should always have a fair chance of perceiving what other party members might be trying to hide from them, but I handle these situations as above in terms of secret knowledge. There are role-playing systems where PVP and inter-party conflict work great; in my opinion Dungeons & Dragons in any of its forms or derivatives are not included in those.
Ultimately, Play The Game You And Your Players Want To Play
This goes back to the idea of the Social Contract. Ask all of your players how they feel about each of these points. Allow them to state their cases, state your own, and attempt to reach a consensus. If there's a disagreement it's up to you to make the final call, which is why it's still worthwhile to hear what other DMs would do in those situations, but again that has to come down to the type of game you'd prefer to run.
