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

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

but otherwise I don't have a clue.