Autocomplete the input
Pyth, 22 13 bytes

mhf!xTd+Qd._z




m         _z  map over prefixes of second input...
 hf    +Qd    find the first element in (dictionary+word) which...
   !xTd       ... starts with k


Sample run:

llama@llama:~$ pyth -c 'mhf!xTd+Qd._z' 
['alpha', 'calf', 'cave', 'coax', 'code', 'codegimmick', 'codegolf', 'cool', 'gamma']
codegolf
['calf', 'coax', 'code', 'code', 'codegimmick', 'codegolf', 'codegolf', 'codegolf']


Thanks to @FryAmTheEggman for 4 bytes!