Skill Ladders
Preparing for a recent Alternity game and I came across something called Skill Ladders whilst reading Wolfgang Baur’s Dataware book. Much of what I learned can be applied to running 4e skill challenges.
Skill ladders are presented by Wolfgang as a way to avoid the monotony of complex skill checks. A complex skill check requires a certain number of successes before a number of failures, just like in 4e. Again just like the OP's problem, this can descend into just totalling successes and failures at the table until you’ve either succeeded or failed.
Wolfgang outlines skill ladders as quickly written lists of what each success or failure means for the characters. Here’s a quick example from my prep for today.
Hacking a locked bio tagged laser weapon
In this example 4 successes are needed before 3 failures
Successes
- Removed take down pin
- Gained access to internal ciruitry
- Bypassed authentication routines
- Weapon unlocked for any user
Failures
- Mild electric shock
- Ammo halved by power discharge
- Weapon still locked. Power discharged. Ammo depleted.
As each success or failure happens then you have a brief thing to tell the players. These don't take long at all to write and you could probably even knock them up in game once you've done a few.
At the start of the skill challenge, if I don't tell the players that
this is a skill challenge, they are likely to mostly propose
role-played actions that are hard to translate into a skill check,
e.g. shouting out "Hold the thief!"
The players can, but don't need to know which skill/ability to roll. Pure RP actions are just fine even if they don't match a skill. You tell them what they can roll when they describe what they're doing, or whether they need to roll at all. Shouting at a crowded marketplace doesn't take much skill, but is unlikely to stop a skilled thief either, so they'll have to follow up with something else. Eventually they figure out they'll need to do better and use their skills. Also, just announcing the beginning of a skill challenge isn't necessarily a bad idea either.
In response to a general "what do you do?" prompt, some players are
more likely to answer than others. Should I impose rules that ensure
that every player gets a turn in the skill challenge and participates?
If this is a problem in your particular group, limiting rolls per player somehow is appropriate. It doesn't have to be either "free rolls" or "turn by turn" - you can decide that the character who succeeded in the last roll is busy carrying out the successful action and can't help with the next roll.
Also don't forget to mix up challenges with different skills; having a Stealth/Thievery/Streetwise -heavy challenge can be done single-handedly by Ron the Rogue if no other rules are imposed.
How do I handle ideas which I like from players, but which don't
really correspond to a skill? Like in the example above, shouting
"Hold the thief!" sounds like a good idea to stop a thief from running
away in a city. But it would translate badly into let's say a
Diplomacy check, especially if the character isn't skilled in that.
If it translates badly, don't translate it. Not everything they do needs to be rolled. Just tell them the action is impossible, or doesn't achieve the desired effect. If the idea is particularly crafty and clever, you can give them a free success. If the idea is particularly bad (tickling the sleeping dragon) you can give them a free failure. It's always up to you.
What if somebody proposes a good idea which corresponds to a skill
that isn't listed in the primary or secondary skills for that skill
challenge?
Let them do it. The primary and secondary skills listed are just suggestions anyway.
what do I do if the players want to cast spells instead of using skills?
It's the GM's call. Some powers have little combat value and yet see use extensively in the RP segments (for example, the Wizard's Cantrip). You can choose to limit the powers your party can use, and it makes sense as it prevents everything from boiling down to combat, but if you do so make sure it's got a sensible reason, like the thief constantly disappearing from view where they're hard to target.
Once the initial spontaneous idea have gotten used up, how do I
prevent the players from simply checking their character sheet for
their best skills and just proposing those?
I don't consider this a huge problem myself. Skill challenges are supposed to be an opportunity for the players to use their best skills. As long as they describe what they're doing, let them go with it. The example in the guide isn't a misuse of the mechanic - for players who don't know the monster manuals and other guides inside-out it's often difficult to remember which knowledge skill (History, Arcana, Religion, Nature...) matches which topic.
Best Answer
Lets start with some caveats, PvP interactions in D&D 4e are poorly defined and effectively mechanized. The second part is that your group's social contract should include the expectations for PvP conflict, the resolution method and whether or not PvP combat is allowed (among other things, there are some good answers about social contracts here if you need more info).
Now, in 4e Diplomacy is always opposed by a DC. However, the DC should be set based on the character's attitude, number of characters influenced and temporary modifiers that depend on what is trying to be accomplished. A suggestion for allowing the opposing player to be involved is to set the DC (a bit lower than you would normally), and allow the Player to roll a check (maybe a Wisdom/CHA Check?) to add to the DC (maybe at 1/2 check).
PvP makes it a bit more difficult to decide a DC. However the amount of leeway given to the DM should allow the DM to decide (based on intended plot) how to set the DC so that either a success or failure is meaningful.
Lastly, I really want to ask (and I wonder this in my group when these situations come up), what purpose does this conflict serve to the narrative? Is it just petty infighting amongst PCs/Players? or does it serve a narrative purpose? If it's enhancing the roleplaying experience for the players than that's great, but if it's just frustrating to all involved then narrate it an move on.