[quote=@The Harbinger of Ferocity]
I might have to comment on one thing in particular, as otherwise the majority of my comments are said by [@BrokenPromise] and it relates explicitly to how collaborative posts are done. The remark is simple, but it is the absolute root of my own disdain for it because while it [i]is[/i] a good tool for character and plot development, I seldom see it employed correctly; I still have not in my three plus years here. So, without further delay, the one addition I have to make.

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One thing I suggest on collab posts is to solely use them when no one else could potentially join in the conversation. For example, an RP that takes place in a setting of an entire city. You call up a friend and go to an ice cream parlor. The likelihood that someone else would magically go to the same parlor is fairly low, unless it is an already-set location in the RP. In the stated case, the conversation does not unfairly omit anyone from the conversation.

An example of a bad time for a collab post: Everyone is gathered together for a party (or after a battle in tabletop style games). You decide that your friend and you will have a conversation together when there are six other people within earshot. Sometimes these conversations are about other people who could overhear, and sometimes they lead to the pair leaving the area for privacy and not allowing for someone to have the opportunity to tag along. 

In my opinion, collab posts can be extremely rewarding in RPs of any sort. I have seen them used almost exclusively in an RP where everyone was okay with the method. Collab posts limit short responses that do little more mechanically than respond to someone else's direct question. If two people have a specific conversation, especially outside of earshot from anyone else, it makes perfect sense for them to work on a post in tandem rather than write a dozen individual posts for the same content (or be cut off because others get annoyed with the dozen posts, and they are asked to cease). But, as Broken Promise mentioned, they are a tool and can be easily abused.