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 ファン outside the stage door. Apart from those (where you'd only be able to have a few 秒 with him) there really is no way ファン 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 あなた are in, if he happenes to be a guest または guest dj on a radio program were あなた can call in and speak with him.

Or, あなた could try yo contact any studio または go through his agent c'mon! all actors have an agent.

but otherwise I don't have a clue.