What's a word for someone who does what they say? [duplicate]

Maybe reliable or trustworthy?

Maybe dependable conveys what you want?

Definition of dependable in English:
adjective

• Trustworthy and reliable:
  'a dependable supply of cold beer'
  'the most valuable and dependable of America’s allies'
  'Growth tends to be mundane with the main attraction being dependable cash flows and reliable dividend payments.'
  'Pierre was the head of all the servants and he was a very reliable and dependable man.'

Reference:
http://www.oxforddictionaries.com/definition/english/dependable

If someone can be described as 'credible', I guess you could assume, we can believe what he/she says to be the truth. Going a bit further, maybe we could also assume that when he/she says something will be done by him/her, then that will be done by him/her definitely.