Character interaction is simply related action between and with multiple characters (PC or NPC). It can be as simple as a conversation.  Or it can be a couple a friends sharing a holonovel. It can also be as complex as a bar fight between 20 people. It is easily the most interesting, fun, and important part of simming as it allows the characters to develop depth and color outside of their bio's.
 

What Types Are There?

The simplest and least interesting type is Mono-Creation (MC). This is when one simmer writes the entire post and uses other characters to help fill out or add realism to the post. When doing this and using another persons PC, the author must practice good sim etiquette and be respectful of the other PC (this is part of the reason behind Bios). Because one simmer can not really speak for another character, such interaction must be limited and fairly neutral (i.e. no making startling revelations about the other PC's past or future intentions). This technique is best used when the simmer is limited in the time s/he has to develop a post or for revelations about ones own character (i.e. writing a letter home).

One of the best ways is through Joint Posts (JP). This is when two or more PC's get together off-line and write a post together. This allows for extensive use of multiple character because their simmers are active in the story creation. This can be accomplished through several means; an OOC request at the end of a post (i.e. 'Hey, does anybody want to do a joint post in the lounge?') or by a private or OOC request to other simmers (i.e. 'Hey, do any of you want to do a joint post where we meet in the Holodeck for a volleyball game?'). A JP allows for in-depth character development of multiple characters, creation of recurring sub-plots (romances, rivalries, recurring jokes...) and therefore really add depth and color to the characters we try so hard to make real. Once some simmers decide to do a JP, it is usually accomplished by one player creating an outline and then having the others fill in their characters parts and suggesting changes to the outline. A potential drawback for this type is if a player doesn't really think the outline is the best, but is wary of suggesting changes because of ego or intimidation issues with the other simmer(s).  Another way to develop a JP is by doing an Off-line Interactive Post (OIP). This is a JP conducted in private or OOC and then once the log is finished it is posted to the entire sim. About the only problem with this is the amount of RL time it takes to develop such a post.

Probably the most interesting, yet most difficult is Interactive Posting (IP). This is when a player simple posts what s/he does or says next, usually covering a very short time interval and then waits for another PC to respond or react. This is somewhat similar to 'chat' sims or traditional table-top RPG's (and is quite similar to URx). With PBeM's this can take a very long time in 'real life' if players do not check and respond quickly to their e-mail, or if a message is delayed to all or some of the recipients. This can lead to some PC's missing an opportunity to act or can leave a situation/scenario hanging for days in RL. One of the real strengths of this type of posting is that no character knows the outcome or reactions of the other players until they are posted and have already happened, this helps add an air of realism and keeps even the posting simmers wondering what will happen next.
 

Examples:
(All examples are reprinted with permission and are copyrighted by the original authors)

Example #1:
This is an example of an MC post:

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=/\="Senior officers, report to the conference room in 5 minutes. Ayasho out."

Making her way onto the bridge, Jen noticed Ensigns McCullum and S'harien manning the bridge. "McCullum, please join us in the conference room. Ensign S'harien, shore leave is canceled, recall all personnel, and make sure the new personnel listed in this mornings roster are included and their gear brought onboard. Make sure Captain da Silva's and Lt Chipmunk's belongings are packed and transferred to the station," she said while striding into the conference room.

Jen took the head chair and waited for her senior staff to show. Tori Ann followed her in and was going to take a place standing near the back of the room when she noticed the Commander motioning her towards a chair with a quick hand gesture.

Once everyone else had arrived (Commander Schouten, Lt Bryce, Lt Matthieu, Lt Gardner, Dr Lowell, Ltjg Milak, Counselor Luann, and Cpt Brinn), Cmdr Ayasho tapped a few buttons on the LCARS terminal and stood up.

"The relevant orders from Command are on your displays. I have been assigned as Captain of the Defender." Walking around the table, the new Captain continued, "Ms. McCullum, please stand. Tori Ann McCullum, on behalf of Starfleet Command, I find great pleasure in promoting you to Lieutenant, Junior Grade," Jen said as she removed the single pip from Tori's collar and replaced it with two. "Congratulations Lieutenant. Also, due to our recent command changes, I am assigning you as acting Chief Tactical Officer."

{snip}

"So, make preparations to get under way in thirty minutes. Lt Bryce, set speed at warp 6, notify me when we are under way. Dismissed"
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This was written solely by the simmer playing Ayasho, who used other PC's but kept their input and actions neutral so as to not inadvertently misuse one of their characters.

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Example #2:
This is an example of a JP. This post was developed using the OIP method, but would not be visually different from one generated using an outline.

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Hearing what Lt. Gardner had to say about changing tactics to better their chances for success intrigues Tori, who has spent a lot of time studying what had happened on the bridge. First looking towards Cpt. Ayasho as if to seek approval then realizing that Ayasho would want Tori to pursue the matter.

"Lt. Gardner, I may ask, what do you mean saying by making different decisions we could of bettered our chances?"

"Well Lt. McCullum, I didn't really wanted to say anything but this: At the time all the decisions Lt-C Lowell took were valid, she had some choices and made the decisions."

"But as for tactics to help us better our chances one I thought was that if we'd have scanned the meteor showers, with deep space probes, we could have found if there was Jem ships in it or not. And if in fact there were no ship in it we could have hidden the Lex in there. Her shield would have been good for staying motionless, not for passing through it."

"On the other hand, if there were ships in the meteor shower than we could have sent you ahead to clear them, since the shuttles are way more manoeuvrable then the Jem ships were it should have been easier to go through them. Also we could have used the probes as a smaller kind of torpedo."

Tori listened as Lt. Gardner spoke being careful not to inject herself until she was sure that Gardner had finished not wishing to show any signs of disrespect.

Closing her eyes Tori tries to think why someone would want to hide the Lexington, but comes up blank.

"Lt. Gardner I don't seem to be able to come up with a purpose for hiding the Lexington in a meteor shower. The Lexington couldn't be repaired in space, the Dominions ships were looking for it and would most likely look in logical hiding places like a meteor shower." "Would not the Jem'Hadar been able to track the emissions from the Lexington's warp engines finding the exact spot at which it entered the meteor shower?" "How long did you plan on leaving the Lexington in the meteor shower?"

Just as she finished another idea popped into Tori's head.

"Had there been Dominion ships on the other side of the meteor shower when you sent the probes through, wouldn't they be able to determine where the probes came from?" "Thus announcing the location of the Lexington and it's escort."

With a deal of anticipation, *I'm sure James had something up his sleeve for doing this, most likely spending hour coming up with something good.* Tori places her elbows on the table interlacing her fingers and resting her chin on her hands waiting for the lieutenant to bedazzle her with his years of experience."

"Well as far as leaving the Lex there, that wasn't what I had in mind, I was thinking more in line of keeping it there until we could assess what was on the other side. As for the probe, type 1 probe have a lot of settings we can use, going from silent running and passive scan to appearing like a larger ship going on it's own way not minding to check what around, to full scan. Also I'm sure Cmdr Lowell would have asked to send them from another bearing, and since they are capable of sell propulsion they could have manoeuvred outside the shower until they were nowhere near our flight path, thus confusing the Jem'Hadar a little more."

"You should also know that the less the speed of a ship the less its emissions [OOC hope I'm not mistaken here!] and since we were traveling under warp 2 emissions would have been pretty low, so then the Dominion ships would have to get around the shower and then scan actively for the Lex, by that time we would have been through the shower."

Looks around the table for any reactions, especially for Matthieu's (about the warp emissions). Then finally looking at Tori thinking *Bright young officer, no wonder that she so brilliantly saved that ferry boat.*

Tori sits back in her chair satisfied with the explanation that Lt. Gardner gave. *I knew that James wouldn't let me down.*

Noting that there is no more question from Tori, James turns to Doctor Lowell for a brief smile.
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This was written by Tori McCullum and James Gardner. If you look carefully at the post you can see how each simmer took turns writing their part and then in the end went back to add a few embellishments (thoughts) in what the other person wrote.