Is it true that...

• all stat checks in dialogs use the protagonist's stats
• all stat checks in scripted interactions allow you to choose which character's stats will be checked

or are there exceptions?

Dialogs ONLY use your PCs stats - always

 

Scripted encounters MAY offer the opportunity to have other characters interact - I would hesitate to say ALWAYS though.

No matter which party member is approached by an NPC, the main character is always the one who answers.  Other party members may make remarks during the conversation but are inexplicably mute whenever the main character is faced with dialogue options, even when they would know an answer and might reasonably be expected to interject at that point in the conversation.  Because of this, it isn't uncommon for non-min-maxing role-players to put more emphasis on Resolve and Lore than they otherwise might.  There are even a few people who've made their main characters ciphers simply because a disproportionate number of class-based dialogue checks are for ciphers.

 

All of that said, we're not talking about Planescape here -- you won't miss much if you design your main character without regard for dialogue. 

Edited September 8, 201510 yr by jsaving

Is it true that...

• all stat checks in dialogs use the protagonist's stats
• all stat checks in scripted interactions allow you to choose which character's stats will be checked

or are there exceptions?

 

There is at least one scripted interaction that only checks your character's stats and equipment - the very first one during the prologue.

I'm pretty sure the options where you can choose who to send base checks on that person's stats.

Yeah, unless it clearly says "Send Eder/other person in", it's checking your main characters stats.