Qsprintf

qsprintf <destination quote> <source quote> <parameter 1> [parameter 2] [parameter 3] ... [parameter 32]

Copies the text of <source quote> to <destination quote>, replacing in order each instance of %d (%ld works too) with a gamevar passed in its parameter position, and each instance of %s with a quote in its parameter position.

You can use a single quote for both placeholders. Also, you can use a single quote to display many different texts at the same time, since using qsprintf in a quote doesn't alter the text displayed by a gametext command used before with that quote.

gamevar TEMP 0 1
definequote 254 PLACEHOLDER
definequote  7 WASTED!
definequote 12 USED STEROIDS 

onevent EVENT_DISPLAYREST
getplayer[THISACTOR].actors_killed TEMP // Get the number of kills the player has

redefinequote 254 ALIENS KILLED: %d    // Redefine a placeholder quote
qsprintf 254 254 TEMP                   // Place our variable into the %d string
gametext STARTALPHANUM 10 10 254 0 0 0 ZERO ZERO xdim ydim // And print to the upper left!

redefinequote 254 %s! %s
qsprintf 254 254 12 7
quote 254 // print: "USED STEROIDS! WASTED!"

endevent