You could just add each person/object to a list, then pick one at random.
Sorta like this.
wholist = NewObjectList ()
whatlist = NewObjectList ()
list add (wholist, MissScarlet)
list add (wholist, ColonelMustard)
list add (wholist, MrsWhite)
list add (wholist, ReverendGreen)
list add (wholist, MrsPeacock)
list add (wholist, ProfessorPlum)
list add (whatlist, Candlestick)
list add (whatlist, Dagger)
list add (whatlist, LeadPipe)
list add (whatlist, Revolver)
list add (whatlist, Rope)
list add (whatlist, Spanner)
index = GetRandomInt ( 0, ListCount ( wholist ) )
game.whodidit = ObjectListItem(wholist, index)
index = GetRandomInt ( 0, ListCount ( whatlist ) )
game.whatdidit = ObjectListItem(whatlist, index)