Because period dress is expensive and prone to heat stroke. These are all valid, canonically acceptable costumes obviously.
Ash Williams - Army of Darkness
You will need:
• Blue button-down shirt
• Khaki pants
• Belt
• Leather shoulder straps
• Chainsaw hand - Make your own for about $20 over here.
• Optional - Boomstick aka fake gun
Your cover story:
While trying to send an evil force back to where it came from, you were pulled through a temporal rift and landed in this strange place. In order to get home, you need to find a book bound in human skin and destroy it. Be sure to ask the actors if they have seen the book or any wandering undead.
Quote to use:
"This is my boomstick!"
Source: reddit.com
Away Team Member - Star Trek
You will need:
• Star Trek t-shirt
• Black slacks or skirt
• Optional - Tricorder
Your cover story:
Your ship was getting highly fluctuating readings from this planet, so you were sent to investigate. Ask around about strange sightings of fairies or other mythical creatures, as they may be manifestations from temporal spikes.
Quote to use:
"Fascinating."
Source: aimeemajor.com
The Doctor - Doctor Who
You will need:
• Pinstripe suit and trenchcoat with optional 3-D glasses
OR
• Button-down shirt, red bow tie and brown plaid jacket with optional fez
OR
• Long striped scarf and brown felt hat with optional velvet jacket
Your cover story:
You are an official royal festival inspector. Just out making the rounds, ensuring proper revelry is being had by all. Be sure to show off your credentials by way of blank paper which is obviously not blank but slightly psychic.
Quote to use:
"Fantastic!"
Source: therpf.com
SG-1 Recruit - Stargate: SG-1
You will need:
• Army fatigues
• SG-1 embroidered badge
• Utility vest
• Optional - Fake gun
Your cover story:
A solar flare interacted with the StarGate wormhole, causing a temporal distortion which flung you into this alternate past. In order to get home, you must find the gate. Ask the actors if they know of any legends surrounding ancient circles in the area.
Quote to use:
"Indeed."
Source: scifihero.net
