Disclaimer: Property of Christopher Nolan and John Twelve Hawks.
Note: Since the timeline for the Traveler is vague (Alice Chen appears to go from being nearly 11 to 12 over the course of what looks like about 8 months) I'm writing it like Mother Blessing started hiding out on Skellig Columba around the time Mal jumped and the events of The Traveler take place immediately after Inception.
A lot of the rules I set up here come from two observations. Eames can pull a PASIV out of a bag that's too small in his dreams but Arthur has to go back for the C-4 in his. Fischer's secret is symbolized by a pinwheel and Saito's is a set of business documents.
The first time Arthur used the PASIV was at Miles's request. Cobb's first impression of Arthur was that the man was too young to look so serious. His second impression was that Arthur had to be the most rigid dreamer in the entire study.
Mal could look at a box and pull anything out of it; mundane things, impossible things, and things that shouldn't fit inside. If it were Cobb's dream he could pull out things that could fit in the box, things he felt ought to be there and sometimes things he imagined might be in the box. Arthur knew what was in his boxes and nothing else could be found in them. If it had a gun in it, then it would always have a gun in it and never a billiard ball or a goldfish or even a different brand of gun.
According to Miles, Arthur's presence in the dream was to deal with projections. The projections were the major hurdle at the stage of exploration that Cobb had reached. He wanted to know what was possible in a dream, why people dreamed in different ways, how far the physics could be bent before they broke and always the study of what secrets these things revealed about dreamer and subject.
Cobb's initial studies with the military had taught him a great deal about the sorts of secrets soldiers carried with them. After a while he could almost always guess a soldier's rank without asking by the way they dreamed. The elasticity of the dreamer's mind and its focus had more to do with their emotional state and upbringing than recent history. Following Miles's suggestions had gained him access to even more information. The trouble was that the information was bound up in symbols known only to the subject. Cobb and Mal knew each other well enough that they could accurately guess the meanings behind the symbols but Cobb wanted something he could apply over a broader range.
He understood why people would be reluctant or unable to tell him exactly what their subconscious' private language meant. It was still frustrating to constantly find himself holding secrets in his hands and have no idea what to put down in his documentation. Even if the secret he retrieved was a blue marble saying that felt wrong. At the same time saying that it might symbolize "something lost in childhood or from childhood" or "a gift given at an important time" was all very vague and made him sound like a quack.
That was why Arthur's first time as the dreamer was such a startling revelation.
Mal opened the safe and pulled out a set of glossy photographs of herself. She laughed as she showed them to Cobb. "This is your secret?"
"That's remarkably straightforward," Cobb squinted at the snap shots of his favorite moments with his fiancee.
Arthur looked at them with polite curiosity.
"They look like real photos," Cobb said, marveling. "Every time we've done this before it was never this clear."
"Arthur has a deep-rooted conviction that a cigar is just a cigar." Mal smiled.
They tested that again and again. Always when Cobb or Mal was the dreamer the symbols were inscrutable but when they tried again with Arthur there were pictures and documents. That was when Cobb started running the tests with the boxes. They found that the flexibility the dreamer showed was reflected in the dreamscape as a whole.
In one of Mal's dreams or one of his own, Cobb could rummage around in his pockets and come up with spare rounds in a hurry. In Arthur's, on the other hand, however many bullets he had was it. It was for Arthur's dreams that Mal and Cobb started planning layouts with extra weapons caches.
That began Arthur's side project; an experiment in guns. In dreams where he could have any gun he wanted it took Arthur a while to settle on what he wanted. Plastic guns, ceramic guns, guns that weren't made anymore, and guns that could be found anywhere. The fascination with firearms wasn't a surprise. Cobb had read Arthur's file. What was a surprise was what happened the first time they were cornered and out of ammunition.
The reason the military training application had fallen through was that repeated exposure to places where danger was literally lurking around every corner turned men into killing machines or destroyed them utterly. The attrition rate was too high for it to be cost effective. Arthur didn't freeze or try to run. He threw himself between Mal and the projections, hitting and kicking. Cobb was gone before the fight finished.
Arthur woke up the same way he always did. One moment he was asleep and the next he was aware and moving.
Cobb watched him carefully. Most of the soldiers who hadn't been able to recover either struggled on waking up or went quieter than usual. "How are you feeling?"
"I'm fine," Arthur looked faintly surprised at Cobb's concern.
"Most people... don't react well to that." Cobb said carefully.
"I'm not most people," Arthur said confidently.