how can i talk to him beside letters

Very occasionally he does fan/autograph signing events, and whenever he does a play, he signs autographs for प्रशंसकों outside the stage door. Apart from those (where you'd only be able to have a few सेकंड्स with him) there really is no way प्रशंसकों can get to talk to him.

As far as I know, David Tennant doesn't have an e-mail account that is opoened to the public.

Neither is his phone number. LOL!

Well, depending on what country आप are in, if he happenes to be a guest या guest dj on a radio program were आप can call in and speak with him.

Or, आप could try yo contact any studio या go through his agent c'mon! all actors have an agent.

but otherwise I don't have a clue.