Physreps, or physical representations of game objects are often used to increase the reality of the game. The most ubiquitous physreps in Assassin games are guns (plastic disk and dart guns usually), thanks to the Guild's origins in Circle-of-Death games of assassination (thus the name). After guns, other sorts of weapons are the most often physrepped items. Unlike guns, though, physreps for swords, knives, etc. are rarely actually used in a weaponlike manner, and exist more to ensure that a character cannot easily conceal his damage-dealing potential. Physrepping beyond weapons is highly variable. In some games, all items are represented by labelled index-cards, with systematic (not always realistic) mechanics for determining how much a character can carry at a time. Most games leave physrepping up to the players, who often choose to provide physreps for their items in order to increase the reality of the game. The GMs will often provide physreps for a few important in-game items which should be distinctive or non-concealable. Still other games, especially the shorter ones, will physrep all game items (though I am currently engaged in writing a tenday game which hopes to do so).
Costuming for Assassin games is by no means required, and opinions on its usefulness vs. the trouble involved vary widely. Characters and their important traits are usually identified by descriptive namebadges or headbands of specified colors, and thus cosuming is not technically necessary. However, many members of the Guild still attempt to costume for many games (particularly the shorter ones). Attempts at costuming range from full-body costumes exact in every detail (usually limited to those easily accomplishable from 20th-century items, such as a suit, hat, and trenchcoat for a 1920's private-eye), to single simple items (such as a hat, sunglasses, or necklace) which cross the line between costume and physrep.
The most important property of Assassin physreps and costumes, in contrast to the costumes and props of a play for instance, is that they are never required to be perfect, but simply to give the impression of what they represent, remind players of what their characters see, and help to enhance the artificial reality of the game. While many players may go to great lengths costuming for a short game or the weekend portions of a tenday game, those costumes often peter down during the week to a single article of clothing worn along with a standard T-shirt and jeans. Duct-tape and flashing LEDs, or random wires and scraps of old circuit boards make a fine representation of cyberware. A baseball cap with the letters "ORB" embroidered on it is a fine representation of a mystical orb circling the wearer's head. A wristwatch on a chain of paperclips does a great impresion of a gold pocketwatch. Lacking for the costuming and props budgets of a play or film, Assassin players do what they can with 5 or 10 dollars at a time. What is important is not the reality, but the way in which an object provides cues which enable the player's imagination to fill in the rest.
A more specific example:
During the week of February 20 through March 1 I was involved in an Assassin game entitled Reign of Terror. The game took place in Paris during the French Revolution, in an alternate-history where Napoleon was killed at the height of his power. My character was a Marshall of France named Andre Massena, a distinguished military man very interested in putting the government back in order. Costuming for that period is not particularly easy on a limited budget and with limited shopping time (I had 4 days notice of what my character would be before the game started). Undaunted, though, I set out to second-hand and craft stores, and came up with enough random bits of costuming and physrepping to be a significant aid in role-playing for myself and others.
The primary element of my costume was a black shirt with epaulets (blue would've been more ideal, but black was what was in my closet). I hung a keychain from one breast pocket, safety-pinned a button from an old jacket to one shoulder, and wrapped some gold-painted rope around the other shoulder to make it look like an appropriately decorated military uniform. On my head I wore a black beret (not necessarily the headware of a 19th century Marshall of France, but it's at least French) with another coat-button pinned on as insignia. I also purchased a second-hand long blue coat which was very appropriate, but used it minimally since Assassin games occur primarily indoors. As far as item physreps, I constructed my Marshall's baton out of a white styrofoam rod and wooden thingie purchased at a craft store, and wore a 2-foot length of foam tubing hanging from my belt on a length of masking-tape as my cavalry saber.
The net effect would certainly never have fooled any 19th-century Frenchmen, but it did give the desired impression. Character recognition is always a difficulty when players jump into the roles of characters who may have known each other from years, and anyone looking at me could in costume could quickly realize at least that I was a military man, and probably a decently high-ranking one, without bothering to consult their "Who's Who in Paris" sheet or the playerlist. My Marshall's baton allowed me to make dramatic gestures, and generally feel more into my role. While I wore the full costume on weekends, during the week I reduced to the hat, baton, and sword-boffer, making it easy for me to switch back and forth from real life to fantasy. I could go from class to class or work on problem sets in my T-shirt and jeans, but once that hat was on my head and the baton in my hand, though, I became Marshall Massena, out to rule France, with 6.371 all but forgotten. Similar bits of costuming and physrepping by other players gave me visual cues to help me to remember who I was talking to, and where I was in the alternate reality we had created. During the week when another character and mine threw a party, we taped the item-card for "a bottle of superb wine" to a bottle of sparkling grapejuice and created a revelry and comraderie as real as any French salon (at least any French salon which containted people with opposing goals, some of whom were quite likely to kill each other within the next few days). The authenticity of the performance thus created would never impress anyone, but the fact that fun was had by all (barring the ever-present frustrations and competition) was unquestionable, making every bit of costuming and physrepping, no matter how silly or low-budget, well worth the effort.